Porous Organic Phenanthroline-Based Polymer as an Efficient Transition-Metal-Free Heterogeneous Catalyst for Direct Aromatic C?H Activation

Article abstract of DOI:10.1002/chem.202100288

Direct C?H bond transformation has been regarded as one of the most important areas in organic synthesis in both academia and industry. However, the heterogeneous transition-metal-free catalysis of direct C?H bond transformation has remained a contemporary challenge. To tackle this challenge, we designed and constructed a porous phenanthroline-based polymer (namely POP-Phen) via free radical polymerization of vinyl-functionalized phenanthroline monomers. POP-Phen shows excellent catalytic performances in transition-metal-free catalyzed C?H arylation, even better than those of the corresponding homogeneous catalyst, which is mainly attributed to the high density of catalytically active sites in the heterogeneous catalyst. Kinetic isotope experiments and spectral characterizations demonstrate the electron-transfer between the heterogeneous catalyst and the base (t-BuOK), a key step for C?H activation. We believe that this porous organic phenanthroline polymer could open a new door for the design of novel heterogeneous transition-metal-free catalysts for direct C?H activation.

Full text of DOI:10.1002/chem.202100288

Accepted Article
Accepted Article
Title: Porous organic phenanthroline-based polymer as an efficient
transition-metal-free heterogeneous catalyst for direct aromatic
C-H activation
Authors: Feng-shou Xiao, Yongquan Tang, Fang Chen, Sai Wang, Qi
Sun, and Xiangju Meng
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202100288
Link to VoR: https://doi.org/10.1002/chem.202100288
01/2020

10.1002/chem.202100288
Chemistry - A European Journal
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Porous Organic Phenanthroline-based Polymer as an Efficient
Transition-metal-free Heterogeneous Catalyst for Direct Aromatic
C-H Activation
Yongquan Tang[a], Fang Chen[a], Sai Wang[a],Qi Sun[b], Xiangju Meng*[a], and Feng-Shou Xiao*[a, b]
[a]
Q. Tang, Dr. F. Chen, S. Wang, Prof. X. Meng, Prof. F.-S. Xiao
Department of Chemistry,
Zhejiang University
Hangzhou, 310028, China.
E-mail: mengxj@zju.edu.cn; fsxiao@zju.edu.cn
Dr. Q. Sun, Prof. F.-S. Xiao
[b]
Key Lab of Biomass Chemical Engineering of Ministry of
Education and College of Chemical and Biological Engineering Zhejiang University
Hangzhou 310027, China
Supporting information for this article is given via a link at the end of the document.
Abstract: Direct C-H bond transformation has been regarded as one
of the most important areas in organic synthesis in both academic
and industry. However, the heterogeneous transition-metal-free
with activated heterocycles promoted by t-BuOK in 2008.[4] Shi,[5]
and Hayashi[6] independently further promoted the development
of this transition-metal-free C-H activation. Since then, a number
of homogeneous catalysts such as proline,[7] imidazolium salts,[8]
alcohols,[9] and phenyl hydrazine[10] have been reported. These
homogeneous catalysts show excellent catalytic performances,
but the difficulty for separating these catalysts from reaction
systems strongly hinders their pratical applications. The solution
for this issue is to develop highly efficient metal-free
heterogeneous catalysts.
Metal-free heterogeneous catalysis are very successful
using graphitic carbons and carbon nitrides already,[11] but the
reactions are mainly focused on oxidative dehydrogenation
(ODH) of unsaturated hydrocarbons and alkane activation,[12]
oxygen reduction reaction (ORR),[13] and the photocatalytic
production of hydrogen.[14] To our knowledge, the attempt for
heterogeneous metal-free activation C-H is very scarce and only
a few successful examples have been reported such as
graphene oxide (GO) and glucose-derived carbonaceous
catalysts, but the low catalytic activities or/and difficult recycling
are trouble for their applications due to the shortage of porosity
and consequently low surface areas.[15]
Recently, great efforts have been devoted to preparation of
porous organic polymers (POPs) such as triphenylphosphine,
1,2-bis(diphenylphosphino)ethane, 2,2,6,6,-tetramethylpiperidine,
and bipyridine.[16] These POPs exhibit abundant and permanent
porosity, high surface areas and pore volume, excellently
thermal and chemical stabilities, flexible molecular design, high
concentration of organic ligands, and superior catalytic
properties.[17] Inspired by this approach for preparation of POPs,
we design and construct the porous organic phenanthroline-
based polymer (POP-Phen) as a heterogeneous catalyst for
direct C-H activation. As expected, the POP-Phen exhibits
excellent catalytic performances and superior recycling ability in
a series of direct C-H arylations.
catalysis of direct C-H bond transformation has been
a
contemporary challenge for a long time. To tackle this challenge, in
this work we designed and constructed porous phenanthroline-
based polymer (namely POP-Phen) via a free radical polymerization
of vinyl-functionalized phenanthroline monomers. This POP-Phen
shows excellent catalytic performances in transition-metal-free
catalyzed C-H arylation, even better than those of the corresponding
homogeneous catalyst, which is mainly attributed to the high density
of catalytically active sites in the heterogeneous catalyst. Kinetic
isotope experiments and spectral characterizations demonstrate the
electron-transfer between the heterogeneous catalyst and the base
(t-BuOK), a key step for C-H activation. We believe that this porous
organic phenanthroline polymer would open a new door for design of
novel heterogeneous transition-metal-free catalysts in the direct C-H
activation.
Direct transformations of saturated, aromatic, olefinic, and
acetylenic hydrocarbons to more valuable products or
pharmaceutically relevant and biologically active compounds
could be regarded as one of the most important processes in the
modern chemistry.[1] As a result, the direct activation of C-H
bonds has drawn much attention for many decades, since it
maximize atom- and step-economy, as well as simplify chemical
synthesis.[2] Notably, transition-metal catalysts have been
involved throughout each stage of the C-H activation over the
past two decades, where the mechanism is mainly followed by
metalation-deprotonation or oxidative addition, which is
dependent on the type and state of the transition metals.[3]
However, the employment of these transition-metal catalysts still
remain challenges such as (i) complicated process for
separation and removal of a trace amount of residual transition-
metals in the final products especially for pharmaceuticals since
many transition-metals are poisonous; (ii) a large amount of
wastes containing transition-metals; (iii) high cost when noble
metals are used. Thus, efficient and transition-metal-free
strategies for direct activation of C-H bonds are always desired.
Metal-free organocatalysis has been around for over a
century, but not long for C-H activation until this decade.[4-10]
Itami and co-workers firstly reported the arylation of aryl iodides
The POP-Phen was prepared from
a
solvothermal
polymerization of vinyl-functionalized 1,10-phenanthroline
monomers (Figure. S1) at 100 C for 24 h in an autoclave
(Figure. 1A, Scheme S1). Figure 1B shows 13C magic-angle
spinning (13NMR MAS) NMR spectrum of POP-Phen, giving a
strong peak at 40.4 ppm, which is assigned to polymerized vinyl
groups. At the same time, disappearance of peaks ranged from
110 to 120 ppm is related to the shortage of viny groups in the
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sample (Figure. S2 ), indicating the complete polymerization
process. IR spectra of the samples show the disappearance of
989 cm-1 and 3009 cm-1 bands associated with viny groups after
polymerization of the monomers, supporting the high cross-link
degree of POP-Phen (Figure. S3 ). Figure 1C shows N2 sorption
isotherms of the POP-Phen, giving the curve of type-I plus type-
IV. The BET surface areas and pore volume of POP-Phen are
estimated at 995 m2g-1 and 1.16 cm3g-1, respectively. A steep
increase in the curve at relative pressure (P/P0<0.01) is due to
filling of micropores, while a hysteresis loop at a relative
pressure of 0.62-0.92 is mainly contributed to the presence of
mesoporosity in the POP-Phen (Figure. S4). The large surface
area and hierarchical porosity are favorable for the mass
transfer in the reactions.[18] Morphology of the POP-Phen is
examined by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) (Figure. 1D and E). The
POP-Phen displays a rough surface that is aggregated with
much smaller particles on the order of tens of nanometers in
size. In addition, the hierarchical porosity of the POP-Phen is
also observed clearly. The power X-ray diffraction (PXRD)
shows a quite broad peak of the POP-Phen, suggesting its
amorphous nature (Figure. S5). Thermogravimetric analysis
(TGA) of the sample is performed under N2 atmosphere,
showing the decomposition temperature of the POP-Phen
started from 430 °C, demonstrating the excellent thermal
stability of the POP-Phen (Figure. S6).
conditions are similar to those reported in the literature (Table
S1).[19] Very interestingly, the POP-Phen catalyzed cross-
coupling of 3-iodoanisole with benzene in the presence of t-
BuOK gives rise to the excellent yield (92.2%) of the desired
product (Table 1, entry 1a). When KOH, K3PO4, K2CO3, or
CH3COOK is empolyed (Table S2, entries 1-4), the POP-Phen is
completely inactive. These results indicate that the t-BuOK plays
an important role for this reaction (Table S2, entry 5).
Subsequently, several control experiments are performed to
exclude the possibility that the reaction is initiated by metal
impurities in the POP-Phen or/and the t-BuOK. When various
available t-BuOK samples are employed, we cannot observe the
change in the product yield; When the reaction is catalyzed by
transition-metal-salts such as PdCl2, FeCl2 and CuCl2 only, the
yield of desired product is very low (<2.3%) (Table S2, entries 7-
9); when the reaction is catalyzed by POP-Phen with 10 mol %
PdCl2, FeCl2 and CuCl2, the yield is higher than 36.4% (Table S2,
entries 10-12); when a new Schlenk tube is used, the product
yield is similar to that of the original one (Table S2 entry13 and
Table 1, entry 1a). In addition, inductively couple plasma mass
spectroscopic analysis (ICP-MAS) of the POP-Phen indicates
that the Pd, Fe and Cu species are undetectable. These results
confirm that the reaction is catalyzed by the POP-Phen rather
than the transition-metals.
A
Figure 2. (A) kinetic isotope effect (KIE) experiment, (B) UV-Vis spectra of the
(a) POP-Phen and (b) POP-Phen/t-BuOK. (C) Electron paramagnetic
resonance (EPR) spectra (X band, 9.82 GHz) of (purple line) t-BuOK (0.3
mmol) in 0.3 mL benzene at room temperature, (blue line) POP-Phen (0.1
mmol) in 0.3 mL benzene at room temperature, and (red line) POP-Phen (0.1
mmol) and t-BuOK (0.3 mmol) in 0.3 mL benzene at room temperature.
To understand the importance of POP-Phen in the reaction,
the experiments for kinetic isotope effects (KIE), UV-Vis, and
EPR techniques have been performed. Notably, the KIE for
benzene is 1.07, indicating that the C-H bond-breaking step is
not the rate-limiting step of the reaction (Figure. 2A and Figure.
S7).[6,20] When the POP-Phen is added into the t-BuOK solution,
it is appeared black color. UV-Vis spectrum of the POP-Phen
(Figure. 2B-a) show absorbance peaks at 226, and 263 nm,
which are attributed to intramolecular charge transfer. However,
after coodinated with t-BuOK, it is shown an obvious red shift
from 306 to 399 nm (Figure. 2B-b), suggesting the
intermolecular charge transfer between the polymer and t-
BuOK.[21] In addition, a mixture of the POP-Phen (0.1 mmol) with
t-BuOK (0.3 mmol) under N2 atmosphere in the presence of
benzene, POP-Phen accepted an electron and transformed into
radical anion to give the EPR signal at g = 2.0044 (Figure. 2C,
Figure 1. (A) Proposed structure, (B) 13C MAS NMR spectrum, (C) N2 sorption
isotherms, (D) SEM image, and (E) TEM image of the POP-Phen.
To evaluate catalytic performance of the POP-Phen as a
heterogeneous catalyst, the reaction of C-H arylation of benzene
with aryl halides as a model is chosen due to its importance for
organic synthesis, as presented in Table 1. The reaction
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red line), which is in good agreement with the electron transfer
process between POP-Phen and t-BuOK.[22] In contrast, when
the POP-Phen or t-BuOK is absent in the system, it is difficult to
observe this EPR signal (Figure. 2C, purple line, and blue line).
When TEMPO (1.0 equiv.) as a radical scavenger is introduced,
this reaction is completely halted (Table S2, entry 14). These
results demonstrate that the radical intermediates should play a
key role for the reaction.
Phen concentration in the heterogeneous catalysts (Table 1,
entries 1a, 1c-e). In addition, liquid adsorption tests showed that
each gram of POP-Phen can adsorb 3.42 g of 3-iodoanisole and
1.58 g of benzene, indicating that the reactants can be easily
enriched in the polymer during the reaction process.
Considering that the key step in the reaction is the formation of
radical species, it is suggested that higher activity of the
heterogeneous POP-Phen catalyst than that of the
homogeneous phenanthroline catalyst might be related to higher
concentration of radical intermediates in the heterogeneous
catalyst than that in the homogenous catalyst. In the
homogeneous catalysis, the phenanthroline species are
uniformly distributed in homogeneous solution everywhere. In
contrast, in the heterogeneous catalysis, all phenanthroline
species just exist in the solid catalysts, only a small part of the
reaction system, as shown in Figure S23. As a result, higher
concentration of the phenanthroline species leads to the
formation of higher concentration of the radical intermediates in
Therefore, it is proposed the reaction mechanism in the
following: The electron transfer takes place between t-BuOK and
POP-Phen that generates compound 1, which activates the aryl
iodides to form the aryl radical 3. After reaction with benzene, it
is given cyclohexadienyl radical 4. Single electron oxidation of 4
by radical cation 2 and then deprotonated by t-BuO- results in
the formation of desired product and t-BuOH. Complex 5 reacts
with t-BuOK to regenerate compound 1, as given in Scheme 1.
This mechanism is well consistent with that catalyzed by the
homogeneous 1,10-phenanthroline catalyst.[6]
the
reaction.[21]
Therefore,
higher
concentration
of
reactants/intermediates are favorable for the formation of
desirable products in the catalytic reactions, in good agreement
with those reported previously.[16a,21,24] The conclusion is also
supported by the experiments of changing the solvent volume
(Table S4).
To illustrate the generality of this arylation, various aryl
iodides are examined (Table 1). Electron-donating substituents
on the phenyl ring of aryl iodides such as methoxyl and methyl
groups provide high product yields (Table 1, entries 2a-7a).
When the aryl iodides contain polysubstituents and electron-
withdrawing
groups
(4-chloro-1-iodobenzene,
4-fluoro-1-
iodobenzene, and 1-iodo-4-(trifluoromethyl)benzene), it is also
obtained excellent yields (Table 1, entries 7a-10a). In contrast,
the position of substituent on the phenyl ring does not strongly
influence the product yields (Table 1, entries 1a-3a).
Since aryl bromides are cheaper and more readily available
feedback than aryl iodides, the development of highly active
transition-metal-free catalysts for direct C-H arylation of benzene
with aryl bromides is of great importance for practical
applications. Very interestingly, both electron-donating and
electron-withdrawing aryl bromides are used as reactants, this
reaction affords high yields of desired products (Table 1, entries
1f, 2b-10b). These results extensively broaden applications of
the POP-Phen catalysts in the future.
The recyclability of heterogeneous catalysts is important for
industrial applications. To evaluate the POP-Phen recyclability, it
is recovered the used catalyst by centrifugating, washing with
water and tetrahydrofuran, and drying under vacuum at 100 °C.
The yield is well maintained after five cycles (Figure. S24),
indicating that the heterogeneous POP-Phen has a superior
recyclability. Furthermore, we performed a gram-scale reaction
of 3-iodoanisole or 4-iodoanisole with benzene, still giving high
yields of the desirable products (Table 1, entries 1a and 2a). In
addition, Figure S25 shows N1s X-ray photoelectron
spectroscopy (XPS) of POP-Phen and the catalyst after
catalysis (POP-Phen-AR), exhinbiting the same binding energy
at 398.9 eV. Moreover, the IR-spectra (Figure. S26) of POP-
Phen and POP-Phen-AR shows the same bands, which indicate
that the aromatics cannot react with POP-Phen during the
reaction. This inference is strongly supported by carbon balance
for reactants and products. After calculation, the carbon balance
is 99.6%. Considering the errors bar (GC analysis), it is
suggested that the arylation of the POP is relatively difficult
compared with benzene, which might related to that the
Scheme 1. A proposed mechanism.
It is worth noting that the catalytic activity (92.2%, Table 1,
entry 1a) in C-H arylation of benzene with 3-iodoanisole over the
heterogeneous POP-Phen catalyst is higher than that (83.2%,
Table 1, entry 1b; 44.1% vs 26.5%, Table S2 entries 15 and 16)
over the homogeneous catalyst. Recently, many studies shows
the heterogeneous catalysts exhibited better catalytic
performance than homogeneous catalysts by constructing the
homogeneous ligands into porous frameworks.[23] In this work,
to understand this unusual phenomenon in this reaction, we
investigated the activation energies of POP-Phen and 1,10-
phenanthroline in C-H arylation of benzene with 3-iodoanisole.
As showed in Figure S8, the POP-Phen shows a little lower
activation energy than the 1,10-phenanthroline, suggesting that
construction of the Phen sites into porous polymers is beneficial
to enhance the activity. To confirm this hypothesis, we also have
prepared heterogeneous catalysts (POP-DVB/Phen-x, x=1.0, 2.0,
and 10.0, see supporting information) with various
phenanthroline
concentrations
in
the
samples
from
copolymerization of divinyl benzene (DVB) monomers with vinyl-
functionalized phenanthroline (DV-Phen) monomers (DVB/Phen
at 1.0, 2.0, and 10.0). The fully characterization of POP-
DVB/Phen-x is presented in supporting information (Figure. S9-
22, and Table S3). Notably, the catalytic activities increase with
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catalyst; [d] POP-DVB/Phen-1.0 was used as catalyst; [e] POP-DVB/Phen-2.0
was used as catalyst; [f] POP-DVB/Phen-10.0 was used as catalyst.
conjugated structure in 1,10-phenanthroline is more stable than
pure benzene ring. Similar phenomena also occur in the
corresponding homogeneous catalysis. N2 sorption isotherms of
the POP-Phen-AR (Figure. S27 and S28; Table S3) shown the
pore structure of POP-Phen can be maintained after reaction.
These results suggest the high stability and the potential
applicability of this catalyst.
In conclusion, we have successfully synthesized a porous
organic phenanthroline polymer (POP-Phen) with high surface
area, hierarchical porosity, and excellent chemical stability. The
POP-Phen exhibits excellent activities and good recyclability in
transition-free-metal catalyzed C-H arylation reaction, which
even gives higher activity than corresponding homogeneous
catalyst due to its high density of catalytically active sites. This
work might offer a new opportunity for construction of novel
heterogeneous transition-metal-free catalysts for the direct C-H
arylation in the future.
Table 1. POP-Phen catalyzed direct C-H arylation of benzene with aryl
halides[a]
.
Entry
Product
X
Yield[b] (%)
Acknowledgements
1a
1b
1c
1d
1e
1f
I
I
92.2 (89.1)
83.2[c]
89.3[d]
82.3[e]
77.0[f]
This work was supported by National Key Research and
Development Program of China (2017YFB0702803) and the
National Natural Science Foundation of China (91945302,
21720102001, U20B6004 and 91634201).
I
I
Keywords: transition-metal-free catalyst • C-H activation• direct
arylation •porous organic polymer
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10.1002/chem.202100288
Chemistry - A European Journal
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We designed and constructed porous phenanthroline-based polymer (namely POP-Phen) via a free radical polymerization of vinyl-
functionalized phenanthline monomers. The resultant POP-Phen has high surface area, abundant and permanent porosity,
excellently thermal and high concentration of organic ligands. This POP-Phen shows excellent catalytic performances in transition-
metal-free catalysed C-H arylation, even better than the corresponding homogeneous catalyst, which is mainly attributed to the high
density of catalytically active sites in the heterogeneous catalyst.
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