Molecular engineered palladium single atom catalysts with an M-C1N3subunit for Suzuki coupling

Article abstract of DOI:10.1039/d1ta00012h

Single atom catalysis has emerged as a powerful technique for catalysis due to its outstanding performance and atom economy. Controlling the hybridization of the atom with its environment is crucial in determining the selectivity and/or yield of the reaction. However, the single atom environment is usually ill-defined and hard to predict because the pyrolysis process used in preparing SACs damages the original status of the precursors in the catalyst preparation. A molecular engineering approach to synthesize single atom catalysts (SACs) on a heterogeneous template provides a strategy to make SACs with a highly uniform coordinating environment. Herein, we report the preparation of a molecular engineered Pd single atom catalyst with a pre-defined M-N3C1 coordination (Pd-N3C1-SAC) using a structure-rigid Pd-N3C1 porphyrin as the precursor, which shows more efficient Suzuki coupling compared with the SAC with Pd-N4 coordination. The origin of the high activity of the Pd-N3C1-SAC is revealed through density functional theory calculations, where a lower reaction barrier for the rate-determining oxidative addition is identified. This journal is

Full text of DOI:10.1039/d1ta00012h

Journal of
Materials Chemistry A
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PAPER
Molecular engineered palladium single atom
catalysts with an M-C N subunit for Suzuki
1
3
Cite this: J. Mater. Chem. A, 2021, 9,
1427
1
coupling†
ab
a
ac
b
d
Jia Liu,
‡
Zhongxin Chen, ‡ Cuibo Liu,‡ Bao Zhang, Yonghua Du,
a
d
e
a
a
af
Chen-Fei Liu, Lu Ma, Shibo Xi,
Runlai Li, Xiaoxu Zhao, Jingting Song,
b
a
b
b
a
Xin Zhi Sui, Wei Yu, Ling Miao,* Jianjun Jiang,
Ming Joo Koh
*
a
and Kian Ping Loh
*
Single atom catalysis has emerged as a powerful technique for catalysis due to its outstanding performance
and atom economy. Controlling the hybridization of the atom with its environment is crucial in determining
the selectivity and/or yield of the reaction. However, the single atom environment is usually ill-defined and
hard to predict because the pyrolysis process used in preparing SACs damages the original status of the
precursors in the catalyst preparation. A molecular engineering approach to synthesize single atom
catalysts (SACs) on a heterogeneous template provides a strategy to make SACs with a highly uniform
coordinating environment. Herein, we report the preparation of a molecular engineered Pd single atom
catalyst with a pre-defined M-N
porphyrin as the precursor, which shows more efficient Suzuki coupling compared with the SAC with
Pd-N coordination. The origin of the high activity of the Pd-N -SAC is revealed through density
3 1 3 1 3 1
C coordination (Pd-N C -SAC) using a structure-rigid Pd-N C
Received 1st January 2021
Accepted 16th April 2021
4
3 1
C
DOI: 10.1039/d1ta00012h
functional theory calculations, where a lower reaction barrier for the rate-determining oxidative addition
rsc.li/materials-a
is identified.
devoted to address this by improving the catalyst design, where
successful examples in SAC-catalysed, selective hydrogenation
Introduction
4,5
Industrial production of commodity and ne chemicals and oxidation were reported. For instance, we have recently
requires a highly selective catalytic process owing to the high disclosed the rst SAC-catalysed preparation of pharmaceuti-
®
7
1
6
demands for purity and cost-effectiveness. Single atom catal- cals (Lonidamine) and late-functionalization (Tamiu )
ysis has witnessed rapid developments in the last decade owing through selective hydrogenation of a-diazoesters to E-
to atom economy, ultrahigh turnover frequency and unique hydrazones and selective oxidation of suldes to sulfoxides
1–3
selectivity. Unfortunately, applications of SACs were largely and sulfones, respectively.
limited to gas phase reactions or electrocatalysis due to the lack
Among various strategies to assemble complex molecules,
of sufficient activation of reactants by single metal sites under carbon–carbon bond formation by palladium-catalysed Suzuki
4,5
liquid phase conditions. Considerable efforts have been coupling remains one of the most important reactions owing to
8–11
its wide applications across the industry.
The uniqueness of
Pd SACs in cross-coupling has recently been demonstrated by
Department of Chemistry, National University of Singapore, 3 Science Drive 3, P ´e rez-Ram ´ı rez et al., where Pd
SACs exhibited an excellent
1
1
a
8
Singapore 117543, Singapore. E-mail: chmkmj@nus.edu.sg, chmlohkp@nus.edu.sg
b
turnover frequency in a continuous-ow setup. Progress on
School of Optical and Electronic Information, Huazhong University of Science and
Suzuki, Ullmann, Sonogashira and Heck couplings had also
Technology, Wuhan 430074, PR China. E-mail: miaoling@mail.hust.edu.cn
9–14
been reported using heterogeneous SACs,
however, efficien-
c
Department of Chemistry, Institute of Molecular Plus, School of Science, Tianjin
cies were generally poorer than homogeneous variants possibly
University, Tianjin, 300072, China
d
0
2+
due to the lack of a dynamic catalytic cycle involving Pd /Pd
states for the oxidative addition-reductive elimination
National Synchrotron Light Source II, Brookhaven National Lab, Upton, New York
1
e
1973, USA
Institute of Chemical and Engineering Sciences, Agency for Science, Technology and pathway.15–21 Regulation of the catalytic center by pyrolysis is
Research, 1 Pesek Road, Jurong Island, Singapore 627833, Singapore
22–24
difficult when using unstable Pd precursors.
This calls for
f
Joint School of NUS and TJU, International Campus of Tianjin University, Fuzhou
a molecularly precise approach to control the catalytic centre,
together with morphological engineering to maximize the
exposure of active sites.
3
†
1
‡
50207, China
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ta00012h
These authors contributed equally to this work.
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Herein, we report a molecular engineered Pd single atom
catalyst with a pre-dened M-N C coordination (Pd-N C -SAC),
3
1
3 1
which shows a higher efficiency compared with SACs using the
Pd-N coordination. The as-synthesized SAC exists in the form
4
of two-dimensional nanosheets, which allows them to be
recovered readily from the reaction mixture and recycled. Owing
to the enhanced electron density at the Pd centre, the Pd-N C -
3 1
SAC was found to exhibit a lower reaction barrier (ꢀ0.31 eV
difference) for Suzuki coupling. Utility was demonstrated by
functionalization of indomethacin to afford derivatives in a late-
stage setting. Such a strategy is universal and can be extended to
Ni, Cu, and Sn-based systems.
Results and discussion
As shown in Fig. 1, our Pd SACs were synthesized by the
pyrolysis of the corresponding Pd precursor (Pd-modied tet-
raphenylporphyrin, Pd-TPP for the Pd-N
N-confused tetraphenylporphyrin, Pd-NCTPP
4
-SAC; and Pd-modied
25–27
for the Pd-
Fig. 2 Characterization of Pd SACs. (a, b) General view and atomic-
resolution STEM-HAADF images for the Pd-N C -SAC and (c) Pd-N -
3 1 4
ꢁ
N
3
C
1
-SAC) and glutamic acid on an MgO template at 600 C
under Ar. Aer removing the template by acid wash, ultrathin SAC; (d) AFM topography and the corresponding line profile of the Pd-
N
3
C
1
-SAC; (e) FT-IR spectra of Pd SACs showing a C]N rich back-
Pd SACs with a typical thickness of ꢀ2.5 nm were isolated as
shown in the AFM images in Fig. 2c & S7.† FT-IR spectra of Pd
SACs exhibited the strong C]N skeleton vibration from the
bone; (f) N adsorption/desorption isotherms at 77 K; and (g) STEM EDS
mapping of the Pd-N C -SAC. Scale bar: (a) 200 nm; (b, c) 5 nm; (d) 2
3 1
mm, (g) 200 nm.
2
ꢂ1
porphyrin structure at 1400 cm together with weak N–H and
C–H vibrations. Such ultrathin morphology maximized the
exposure of surface active sites to reactants owing to shortened
of TPP by NCTPP did not affect the dispersion of Pd atoms
owing to abundant N ligands in the conjugated structure of
porphyrin, whose uniform distribution was observed as bright
28,29
diffusion pathways.
showing ultrahigh surface areas of 1025 and 832 m g from
BET measurements of the Pd-N C -SAC and Pd-N -SAC with
Our Pd SACs were also microporous,
2
ꢂ1
3
1
4
8
spots as shown in Fig. 2b and c. This was conrmed by the
typical pore sizes of 0.5 and 1.4 nm, respectively. Detailed
characterization of the chemical composition of precursors and
SACs by UV-Vis, NMR, Raman, and FT-IR can be found in the
ESI.†
We rst characterized the atomic dispersion of Pd by scan-
ning transmission electron microscopy in the annular dark eld
mode (STEM-ADF). The as-synthesized Pd SACs exhibited a two-
dimensional nanosheet morphology (Fig. 2a). The replacement
ꢁ
absence of Pd (111) and (200) peaks at ꢀ40 and 45 in the XRD
spectra. The Pd mass loading was determined to be 3.3% for the
3 1
Pd-N C -SAC by inductively coupled plasma (ICP) analysis
shown in Table S1.† The existence of Pd single atoms was
further supported by energy-dispersive X-ray spectroscopy (EDS)
elemental mapping in Fig. 2g and additional images in Fig. S8–
6
S13.†
The substitution by Pd–carbon bonds in the N-confused
porphyrin and Pd-N
electronic structure.
3
C
1
-SAC has a profound inuence on the
The X-ray photoelectron spectra (XPS)
25–27
of Pd3d core-level displayed in Fig. 3a reveal that the charac-
2
+
teristic peaks of Pd 3d3/2 and 3d5/2 are negatively shied to
lower binding energies of 343.3 and 338.0 eV in the Pd-N C -
3
1
SAC, respectively, indicating valence states that are between
1
+
2+
Pd and Pd , which are lower than that of the typical
15,25
porphyrin-like structure.
The conguration of N in Pd SACs
was also veried by N1s core-level XPS spectra shown in Fig. 3b,
in which pyrrolic N at 400.6 eV from N-confused porphyrin and
pyridinic N at 398.9 eV from the pyrolysis of glutamic acid were
8,15
dominant. X-ray absorption spectroscopy (XAS) is a powerful
technique to distinguish the electronic and coordination
1
structure of SACs. As shown in Fig. 3c, the extended X-ray
absorption ne structure (EXAFS) spectra of Pd SACs exhibit
˚
a characteristic Pd–N peak at 1.4 A that originates from the
porphyrin precursors. This is distinct from the metallic Pd–Pd
Fig. 1 Preparation of two-dimensional Pd SAC nanosheets with
a precise M-N C subunit for Suzuki coupling.
3 1
˚
peak at 2.5 A in Pd foil, suggesting the atomic dispersion of Pd
11428 | J. Mater. Chem. A, 2021, 9, 11427–11432
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chemoselective Suzuki coupling of C–I bonds and heterocyclic
substrates (benzodioxole, pyridines and thiophene that may
poison the Pd catalyst by the coordination with N or S atoms)
(Fig. S16†). For the 1,3,5-triiodobenzene substrate, the corre-
sponding 1,3,5-triphenylbenzene was produced in quantitative
yield. Notably, 9-cyanophenanthrene with potential utility in
medicines can be generated in a one-pot reaction involving
30
tandem Suzuki coupling and cyclization.
Further catalyst screening was conducted using the less
activated bromobenzene (Fig. 4), where conventional hetero-
geneous catalysts such as 10% Pd/C showed an inferior
performance compared with the homogeneous catalyst
Pd(PPh
a highly competitive yield (ꢀ90%) comparable to Pd(PPh
outperformed the Pd-N -SAC (10% yield). This could be attrib-
uted to the lower valence state and greater electron density of
3
)
4
(8 vs. 94%). Notably, our Pd-N
3
C
1
-SAC exhibited
3 4
)
, and
4
23
3 1
the Pd-N C -SAC. The catalytic activity was strongly tempera-
ture dependent judging from the plot of yield versus tempera-
ꢁ
ture in Fig. 4b. Increasing the temperature from 90 to 110 C
increased the catalytic yield of the Pd-N C -SAC from 10% to
Fig. 3 Structural characterization of Pd SACs. (a) High-resolution XPS
3 1
ꢁ
Pd3d and (b) N1s spectra of Pd SACs; (c) Pd K-edge FT-EXAFS spectra of 90%. A further increase to 120 C resulted in a lower yield (75%)
Pd foil, Pd precursors and Pd SACs; (d) XANES spectra with enlarged as a result of undesired homocoupling of boronic acids.
images of the edge structure and white-line. The inset shows the
corresponding EXAFS spectra.
8
Compared to the seminal research by P ´e rez-Ram ´ı rez et al., our
Pd-N
3
C
1
-SAC has a higher TOF value upon completion of the
ꢂ1
reaction (3.8 vs. 1.9 h , batch mode) and is easier to prepare
than related homogeneous or supported porphyrin catalysts.
34,35
8
in our catalysts. The intensity of the Pd–N shell gradually
decreases from Pd–TPP (precursor of the Pd-N
4
-SAC) to the Pd-
3 1
N C
-SAC, as a result of the breakage of structural symmetry by
carbon substitution and disturbance of the planar aromatic
structure in the porphyrin scaffold aer pyrolysis. Further
analysis by EXAFS tting shown in Fig. S14 and Table S2†
conrmed a slightly lower coordination number of ꢀ3 in the Pd-
3 1 3 1
N C -SAC. Meanwhile, the oxidation state of Pd in the Pd-N C -
SAC is estimated from the Pd K-edge X-ray absorption near edge
structure (XANES) spectra shown in Fig. 3d, in which a slightly
lower oxidation state (<+2) compared with porphyrin precursors
and the unmodied Pd-N -SAC is revealed in the edge structure
4
and white-line intensity; the oxidation state has also been
23
conrmed from XANES simulation results shown in Fig. S15.†
Currently, most of the coordination structures were deduced
22–24
from EXAFS tting or calculations.
Pd SACs constructed
from a molecularly precise precursor provide a well-dened
environment for studying the dynamics of the metal–ligand
8
coordination during the course of the reaction.
We began with a standard Suzuki coupling reaction using
iodobenzene and 4-methylphenylboronic acid with 1 mol% Pd
catalyst, as shown in Fig. S16.† Both Pd SACs exhibited nearly
quantitative conversion to the desired product, which was
comparable to homogeneous catalysts Pd(PPh
3
)
4
in terms of
ꢂ1
Fig. 4 Pd SAC-catalysed Suzuki coupling using bromo-compounds.
turnover frequency (TOF ¼ 4.2 h ), and outperformed 10% Pd/
(
1
a) Catalyst screening. The inset shows the model reaction using
C and Pd(PPh
stability without any performance decay aer 5 cycles. To
demonstrate synthetic utility, a wide range of substrates were stability and (d) kinetic study of the Pd-N C -SAC toward Suzuki
3
)
2
Cl
2
. Our Pd SACs displayed good cycling
ꢁ
mol% Pd catalyst and 3 equiv. of K
2
CO
3
in DME at 110 C for 24 h; (b)
temperature dependence on catalytic performance; (c) cycling
3
1
coupling. The inset shows the representative STEM image of the spent
catalyst; (e) demonstration of functional group compatibility in the
examined including those that are problematic using homoge-
8–11
neous catalysts,
including electron-rich and electron-
, –NO and acetyl),
3 1
synthesis of indomethacin derivatives using the Pd-N C -SAC. Scale
decient units on the aryl ring (–OMe, –CF
3
2
bar: 5 nm.
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Our study shows that SACs coordinated in a N-confused struc-
36,37
ture are powerful catalysts.
Kinetic studies were conducted to provide insights into the
reaction mechanism (Fig. 4d). We found that the Pd-N C -SAC
3
1
4
was a superior catalyst to the Pd-N -SAC and Pd/C in view of its
much faster reaction rate. A short induction period (ꢀ6 h) was
observed due to the complicated pathway (dynamic valence
state changes of Pd SACs) to produce the coupling product.
Further verication by hot-ltration experiments ruled out the
possibility of the dissolution of the Pd SAC into homogeneous
8–11
species.
No catalyst leaching into the supernatant of the
reaction mixture was observed by ICP-OES. This was conrmed
by the stability measurements shown in Fig. 4c, where our Pd-
N C -SAC exhibited excellent catalytic performance for bromo-
3 1
benzene without signicant decay aer 5 cycles. The additional
STEM image in the inset of Fig. 4d validated the atomic
dispersion of the spent catalyst. The two-dimensional nano-
sheet structure of the Pd-N C -SAC maximizes the surface area
for the reaction and its micron-size lateral dimension allows the
catalyst to be separated from the reaction medium readily by
Fig. 5 DFT calculations of the reaction pathway. (a) Charge densities
4 3 1
of the Pd-N -SAC and Pd-N C -SAC; (b) projected density of states
3
1
(PDOS) for Pd, N and C for Pd SACs; (c) energy diagram and reaction
pathways for the phosphine-free Suzuki coupling reaction between
bromobenzene and phenylboronic acid in the presence of Pd SACs.

ltration, unlike more tedious separation needed for homoge- Base and outer carbon atoms in the model are omitted for the sake for
clarity. Color scheme used: brown for C, light blue for N, grey for Pd,
blue for Br and light-pink for H.
neous catalysts. Finally, to highlight the exceptional functional
group compatibility, we had successfully transformed
a substrate derived from multifunctional indomethacin, an
FDA-approved anti-inammatory drug, to its long-chain
aromatic ester derivative in 84% NMR yield through sequen-
tial esterication and Suzuki coupling in the presence of the Pd-
N C -SAC (Fig. 4e). This validates the usefulness of heteroge-
This is in accordance with a lower bandgap (ꢂ1.54 eV) and
a lower valence state in XPS. The free energy diagrams of Suzuki
coupling in Fig. 5c, S20 & S21† (base is omitted for the sake for
3
1
clarity) reveal a similar adsorption barrier for the Pd-N -SAC and
neous single atom catalysts for the production of high-value
chemicals.
4
the Pd-N C -SAC at the beginning of the reaction, while
3
1
subsequent activation of bromobenzene occurs with a much
Our molecular engineering method can also be used to
fabricate Ni, Cu, and Sn-based SACs (Fig. S19†), these are more
8
higher barrier for both and is rate-determining. The reaction
31–33,38
3 1
barrier for the Pd-N C -SAC is signicantly lower than that of
sustainable alternative to Pd-based catalysts.
We have
the Pd-N
4
-SAC (0.31 eV difference) owing to its lower valence
examined the use of nonprecious Ni-based SACs in Suzuki
coupling, which has not been reported to date. As shown in
state and enhanced electron density on the Pd center to facili-
39–41
tate oxidative addition.
This was also reected in the facile
Fig. S19,† by using the Ni-N -SAC and iodobenzene, ꢀ78% of
4
nucleophilic attack of the oxidation addition intermediate to
phenylboronic acid, showing a more exothermic process on the
the desired product can be obtained aer preliminary optimi-
zations. However, lower yields (60–32%) were observed for
substrates containing electron-withdrawing groups. The Ni-
surface of the Pd-N
transmetalation of boronic acid with the assistance of the base
is endothermic, where the Pd-N -SAC also exhibits a dimin-
3 1 4
C -SAC than the Pd-N -SAC. Subsequent
3 1
N C -SAC also gave an unsatisfactory yield of 24% for the model
3 1
C
reaction. Although further catalyst design is necessary to
improve the catalytic efficiency, our work demonstrates the
viability of metal-derived SACs in promoting important C–C
bond forming transformations.
ished energy barrier. Finally, the C-C bond is formed aer
reductive elimination, delivering the desired product and re-
39–41
generating the catalyst.
tions, we conclude that the lower reaction barriers associated
with the Pd-N C -SAC-catalysed sequence likely translate to
Based on the above DFT calcula-
Density functional theory (DFT) calculations were carried out
to investigate the effect of changing from a N
4
9–41
to N
3
C
1
coordi-
In Fig. 5a, the
3 1
C -SAC were analyzed
3 1
3
better catalytic performance in the cross-coupling of
bromobenzene.
nation environment on the Suzuki coupling.
charge density of the Pd-N -SAC and Pd-N
4
to identify the valence state of the Pd atom. Due to the different
electron withdrawing properties of carbon, the Bader charge of
Pd in the Pd-N C -SAC is lower than that of the Pd-N -SAC (+0.59
Conclusions
3
1
4
vs. +0.72), which reects its lower oxidation state. Moreover, the In conclusion, the molecular engineering of two-dimensional
projected density of states (PDOS) for the metal center and its Pd SACs using porphyrin precursors produced a unique M-
coordinated N atoms were calculated to reveal the differences in N C coordination in the Pd single atom, which led to a signif-
3
1
the electronic structure of the active site. As shown in Fig. 5b, icant improvement in the efficiency of SAC-catalysed Suzuki
compared to the Pd-N -SAC, the d band of the Pd-N -SAC is coupling. The two-dimensional nanosheet structure of Pd-N
nearer to the Fermi level, which is indicative of higher reactivity. SACs maximizes the surface area for the reaction and they can
4
3
C
1
3 1
C -
11430 | J. Mater. Chem. A, 2021, 9, 11427–11432
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The ability to control the hybridization of the single atom in
a molecularly precise manner allows the catalytic mechanism to
be studied in a well-dened manner; this paves the way forward
to make more potent and atom-economic catalysts.
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Author contributions
J. L. conceived the research, synthesized the materials, con-
ducted catalytic reactions, and wrote the dra with K.$P. L., Z.
C. and C. L. TEM, STEM characterization, and data analysis
were conducted by J. L., and Z. C.; S. X., Y. D. and L. M. per-
formed the EXAFS and data processing; W. Y. and X. Z. per-
formed the XPS and Raman measurements; J. S. and X. S.
performed the FTIR and XRD measurements. B. Z. and L. M.
assisted in DFT calculations. C. L., and R. L. assisted with
materials characterization and data analysis. M. J. K. and
K.$P. L. supervised the research. All authors discussed and
commented on the manuscript.
1
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Conflicts of interest
1
1
There are no conicts to declare.
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J. L., Z. C. and C. L. contributed equally to this work. K. P. L.
acknowledges NRF-CRP grant “Two-dimensional Covalent
Organic Framework: Synthesis and Applications”. Grant
number NRF-CRP16-2015-02, funded by National Research
Foundation, Prime Minister's Office, Singapore. M. J. K. thanks
the National University of Singapore Academic Research Fund
Tier 1: R-143-000-A77-114. L. M. thanks the nancial support
from National Natural Science Foundation of China (51302097
and 51571096). J. L. was supported by the Chinese Scholarship
Council (CSC). This research used 7 of the National Synchrotron
Light Source II, a U.S. Department of Energy (DOE) Office of
Science User Facility operated for the DOE Office of Science by
Brookhaven National Laboratory under Contract No. DE-
SC0012704.
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