Dipolar HCP materials as alternatives to DMF solvent for azide-based synthesis

Article abstract of DOI:10.1039/d1gc02002a

Hypercrosslinked polymers HCP-DMF and HCP-DMF-SO3H containing abundant and flexible DMF moieties were designed and synthesized. Benefitting from the solvation microenvironment provided by the pseudo-DMF moities, the polar HCPs manifested outstanding performances in the conversions of NaN3 to benzylic azides and 1,2,3-triazoles in EtOH (95%), respectively, avoiding the use of risky DMF and improving the separation processes of the products.

Full text of DOI:10.1039/d1gc02002a

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Dipolar HCP materials as alternatives to DMF
solvent for azide-based synthesis†
Cite this: DOI: 10.1039/d1gc02002a
a,b,c
Feng Gao,a Rongxian Bai,a Minghao Li *a and Yanlong Gu
*
Received 6th June 2021,
Accepted 10th August 2021
DOI: 10.1039/d1gc02002a
rsc.li/greenchem
Hypercrosslinked polymers HCP-DMF and HCP-DMF-SO3H con- Despite the fact that biobased solvents γ-valerolactone (GVL),
taining abundant and flexible DMF moieties were designed and dihydrolevoglucosenone (Cyrene™) and dimethyl isosorbide
synthesized. Benefitting from the solvation microenvironment pro- (DMI) have been proved to have the ability to replace DMF in
vided by the pseudo-DMF moities, the polar HCPs manifested out- many organic reactions,13–16 their high boiling points and easy
standing performances in the conversions of NaN3 to benzylic miscibility with water result in wastewater discharge and
azides and 1,2,3-triazoles in EtOH (95%), respectively, avoiding the energy-intensive processes for recycling. In addition, solid-
use of risky DMF and improving the separation processes of the phase peptide synthesis (SPPS) also suffers from similar trou-
products.
bles when some alternatives to DMF are used such as
N-butylpyrrolidinone (NBP) and binary solvents DMSO/DOL
(1,3-dioxolane) and DMSO/2-Me-THF.17–19
The activities and selectivity of organic conversions are signifi-
cantly influenced by the choice of reaction media. Traditional
dipolar aprotic solvents (DMSO, DMF, DMAc, NMP, etc.) play
an important role in rendering outperformance for a wide
range of reactions including the SN2 reaction,1 Pd-catalyzed
coupling reactions,2–4 conversion of azides5–8 to biomass valor-
ization.9 However, wastewater and high energy input accom-
pany the product separation and solvent recycling due to the
utilization of such a class of high boiling solvents.10 When it
comes to DMF, there are more problems, resulting from more
CMR issues (carcinogenic, mutagenic, or reprotoxic) as com-
pared to DMSO. It has also been reported that DMF has more
potential safety risks in chemical processes because it is
incompatible with a large variety of substances, producing di-
methylamine as well as gaseous products CO2, CO and H2,
leading to many incidents over the years.11,12 Being aware of
these hazards, it is appealing to the scientific community to
identify and develop safer and greener alternatives to DMF.
Creating a local solvation environment for the active sites
in heterogeneous catalysts provides potential and opportu-
nities to conduct some organic reactions in low boiling sol-
vents, thus allowing us to avoid the use of conventional
dipolar aprotic solvents. For instance, porous polymer solvents
(PPSs) with a DMSO moiety and acid COFs incorporating flex-
ible PVP in channels promote the dehydration of fructose to
HMF efficiently in THF rather than in DMSO, which is always
required in common processes.20–22 However, the complicated
design of catalysts and the use of hazardous solvents, like
THF, reflect their limitations in multitudinous organic reac-
tions to realize green prospects. Hypercrosslinked polymers
(HCPs) have been widely used in many applications, such as gas
separation and conversion,23,24 catalysis,25 and energy storage,26
because of their low cost and ease of preparation from various
organic monomers.27 Benefiting from permanent porosity, high
stability and modifiable functionality,28 HCPs-based catalysts
exhibit multipurpose functions in organic synthesis. Indeed,
HCP materials enabled us to easily introduce diverse active sites
and local microenvironments in the catalysts, facilitating the
model transformations including acid–base tandem reactions29
and a variety of name coupling reactions.30 The high propensity
of HCP materials for solvent uptake also ensured they were suit-
able precursors to design heterogeneous catalysts equipped with
a solvation microenvironment.31
aKey laboratory of Material Chemistry for Energy Conversion and Storage, Ministry
of Education, Hubei Key Laboratory of Material Chemistry and Service Failure,
School of Chemistry and Chemical Engineering, Huazhong University of Science and
Technology, 1037 Luoyu road, Hongshan DistrictWuhan 430074, China.
E-mail: klgyl@hust.edu.cn, liminghaochem@hust.edu.cn
bSchool of Chemistry and Chemical Engineering, Shihezi University, Shihezi,
Xinjiang 832003, China
cState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Lanzhou 730000, China
The synthesis and conversion of azides is a common route
to support triazoles and tetrazoles which are two significant
scaffolds in numerous bioactive molecules and functional
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc02002a
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materials.8,32,33 NaN3 is the most commonly used starting
material to obtain organic azides and further construct tri-
azoles and tetrazoles compounds.34,35 However, in the syn-
thesis of these compounds, the habitual use of DMF as a reac-
tion medium for these processes results in a series of issues as
previously mentioned.5,6,36 Therefore, more effort should be
made to minimize and avoid the use of DMF in a much
greener strategy for the transformation of azides.
Based on the readily designable and swelling properties of
HCP materials, in this work, we tried to design dipolar HCPs
with a flexible DMF moiety as solid PPSs with an expectation
to create a dipolar microenvironment for promoting reactions.
Ideally, with the aid of this special HCP material, greener sol-
vents can be used as alternatives to DMF in the conversion of
NaN3 to benzylic azides and triazoles.
Polar HCP materials with a DMF moiety were prepared
according to the route shown in Scheme 1. N-Methyl-N-(3-phe-
nylpropyl)formamide M1 was used as the monomer to obtain
HCP-DMF via a hypercrosslinking reaction. The following sul-
fonation reaction using acetyl sulfate can convert HCP-DMF to
HCP-DMF-SO3H, which was expected to act as an acid catalyst
with a dipolar microenvironment for active sites. The long side
chain in the aromatic ring of monomer M1 ensures HCP-DMF
Fig. 1 (a) The FT-IR spectra of HCP-DMF and HCP-DMF-SO3H; (b) flu-
orescence emission spectra of Nile red adsorbed in different HCP
materials; (c) TGA curves of HCP-DMF and HCP-DMF-SO3H; (d) solid-
state 13C NMR; (e) C 1s and (f) N 1s XPS spectra; and SEM images of (g1)
HCP-DMF, (g2) HCP-DMF-Ben, (g3) and HCP-Ben.
has flexible DMF moieties. In HCP-DMF-SO3H, the accessibil- acidification.37 The existence of –SO3H on the HCP-DMF-SO3H
ity of the DMF moieties to the active acid sites is also expected, is also indicated by a weak peak at 1035 cm−1 due to the
maximizing the positive effect of the local environment on the C(aromatic)–S stretching vibration.38 Nile red (NR) has been
catalytic reaction. For comparison, the two materials of widely used as a probe to determine the polarity of microenvir-
HCP-Ben and HCP-DMF-Ben were also prepared in order for onments in various heterogeneous materials via its fluo-
us to investigate the influence of the DMF moieties content. rescence spectra.39–41 The fluorescence emission spectra of NR
The first one was fabricated using benzene M2 as the adsorbed in different HCP materials were used to estimate the
monomer and the second one was prepared by using a mixture polarity of prepared HCPs, as shown in Fig. 1(c). Upon photo-
of M1 and M2 as the monomers.
excitation at 420 nm, the fluorescence intensity of NR
The DMF moieties in HCP-DMF and HCP-DMF-SO3H were adsorbed in HCP-Ben, HCP-DFM-Ben and HCP-DMF became
demonstrated by the FT-IR spectra as described in Fig. 1(a). obviously weaker and almost disappeared with the increasing
The conspicuous peaks at about 1662 cm−1 both in HCP-DMF content of monomer M1; a slight red shift from 636 nm to
and HCP-DMF-SO3H were attributed to the stretch vibration of 647 nm in the fluorescence maximum of the NR was also
the CvO bond of DMF moieties, which correspond to that of noticed. The weakened intensity and red shift can be ascribed
M1, obviously suggesting the successful synthesis of HCP-DMF to the stabilized twisted intramolecular charge transfer state
and the preservation of DMF moieties even after sulfonation to (TICT) of NR by a polar environment.42 Such a fluorescence be-
obtain HCP-DMF-SO3H. Compared with HCP-DMF, the emer- havior of the dye demonstrated that the polarity of HCP-Ben,
ging two peaks at 1220 cm−1 and 1188 cm−1 in HCP-DFM-Ben and HCP-DMF was enhanced gradually because
HCP-DMF-SO3H were related to the stretching vibration of the of the decoration of the DMF moieties. The thermal stability of
SvO bond of aromatic sulfonic acid, indicating the successful both HCP-DMF and HCP-DMF-SO3H were inspected by
Scheme 1 A schematic illustration of polar HCPs in terms of their structure and preparation.
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thermogravimetric analysis (TGA), as shown in Fig. 1(c). It was highest SBET. Reasonably, the swelling degree of HCP-DMF-Ben
observed from the TGA curves that the weight of two kinds of and HCP-DMF sharply reduced to 150% and 67%, respectively.
HCP materials was maintained until the temperature was elev- In fact, the low weight increase of HCP-DMF suggests that
ated to about 140 °C and the initial decomposition tempera- HCP-DMF has hardly any swelling ability in EtOH (95%) sub-
ture was nearly 200 °C. The TGA results implied that the pre- jected to its low SBET. Because of the deficient swelling degree
pared materials have a good thermostability to be used under of HCP-DMF, the predominant contribution of the DMF moi-
mild conditions. Solid-state 13C NMR and XPS were employed eties to the polar solvation microenvironment could be
to investigate the situation of the DMF-like moieties and the expected, thus facilitating the maximization of the solvent
sulfonic moieties, the spectra are shown in Fig. 1d–1f. It is effect of the supporting materials.
observed from Fig. 1d that the carbon peak at 164 ppm
Nucleophilic substitution of NaN3 to benzyl bromides is a
ascribed to –N-CHO emerged and increased with the addition usual pathway to synthesize benzylic azides as an important
of monomer M1. Besides, the unaltered chemical shift of C in intermediate to triazoles, it is always performed in DMSO or
HCP-DMF-SO3H and HCP-DMF-Ben-SO3H indicated that sulfo- DMF without the addition of a catalyst at high temperature for
nation did not have an impact on the DMF-like moieties, a long time.5,6,43,44 With the polar HCPs in hand, we then
which is consistent with the FT-IR spectra results. Such an examined their function in the replacement of DMF by using
interpretation is also reinforced by the N 1s XPS spectra the reaction of NaN3 to p-methylbenzyl bromide 1a to syn-
(Fig. 1(e)), in which the shape and binding energy of N 1s thesize p-(azidomethyl)toluene 2a. As shown in Table 2, in the
remain after the sulfonation of HCP-DMF to HCP-DMF-SO3H. presence of HCP-DMF, various green solvents, including
The C 1s peak could be resolved into main three peaks as anisole, isopropyl acetate (iPrOAc), 2-methyltetrahydrofuran
shown in Fig. 1(f), in which the binding energy of CvO (287.3 (2-MeTHF) and ethanol (EtOH), were screened for the SN2 reac-
eV) in HCP-DMF-SO3H and HCP-DMF-Ben-SO3H shows no tion at ambient temperature firstly (entries 1–5). It was found
obvious change. All the results disclosed by the 13C NMR and that this reaction cannot proceed in anisole (entry 1) and only
XPS spectra suggest that although DMF-like moieties were suc- trace product was observed by TLC with a much lower conver-
cessfully introduced and remained after sulfonation of sion of starting material in iPrOAc and 2-MeTHF (entries 2 and
HCP-DMF, there is almost no interaction between the DMF- 3). In anhydrous EtOH, the yield of 2a reached 27%, which is
like moieties and –SO3H at the site-isolation state in quite promising and deserves further optimization.
HCP-DMF-SO3H.
Considering the low solubility of NaN3 in anhydrous ethanol,
The scanning electron microscopy (SEM) images disclosed EtOH (95%) was used then as reaction medium. To our
clearly that the morphologies of the three classes of HCPs delight, an excellent yield of 2a (92%) was obtained in the pres-
changed from the dense and blocky texture of HCP-DMF ence of HCP-DMF (entry 5). However, only 69% 2a was separ-
(Fig. 1g1) to the loose and porous structure of HCP-Ben ated in EtOH (95%) without adding the HCP-DMF component
(Fig. 1g3) with the decrease of monomer M1 in the HCPs, (entry 6). When HCP-DMF-Ben and HCP-Ben were added into
while both a compact and lax structure was presented in the reaction system, the yields of the target compound gradu-
HCP-DMF-Ben (Fig. 1g2). The –SO3H concentration of ally reduced from 78% to 67% (entries 7 and 8) mainly caused
HCP-DMF-SO3H, HCP-DMF-Ben-SO3H and HCP-Ben-SO3H was by the fading dipolarity of the corresponding HCP materials
1.2 × 10−3, 1.2 × 10−3 and 1.0 × 10−3 mmol mg−1, respectively, (Fig. 1(b)). The comparable yields of 2a in entry 8 and entry 6
which were calculated according to the S content (see ESI†) indicated that HCP-Ben hardly has any influence on the reac-
determined by elemental analysis (EA).
tion process. All the above results reflected that the introduc-
We also investigated the specific surface SBET and swelling
property of three kinds of HCPs and the results are tabulated
in Table 1. It was noticed that the SBET of the HCP materials
dramatically decreased from 1266.7 m2 g−1 of HCP-Ben to
0.9 m2 g−1 of HCP-DMF with the increase of M1 content. After
sulfonation of HCP-DMF, the SBET of HCP-DMF-SO3H is kept
at 1.1 m2 g−1. There are almost no pores in HCP-DMF. The
changed SBET of different HCP materials were consistent with
the SEM images in Fig. 1. The swelling degree Q of three kinds
of HCP materials in EtOH (95%) was determined as shown in
Table 1. HCP-Ben has the largest Q (603%) benefitting from its
Table 2 The optimization of benzylic azides synthesis from NaN3 and
benzyl bromidea
Entry
Solvent
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
Anisole
HCP-DMF
HCP-DMF
HCP-DMF
HCP-DMF
HCP-DMF
None
NR
Trace
Trace
27
92
69
iPrOAc
2-MeTHF
EtOH
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
Table 1 Properties of different HCP materials
HCP-DMF-Ben
HCP-Ben
78
67
Material
HCP-Ben
HCP-DMF-Ben
HCP-DMF
SBET (m2 g−1
Q (%)
)
1266.7
603
50.6
150
0.9
67
a 1a, 0.3 mmol, NaN3, 0.35 mmol,1.5 h, isolated yields.
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a
Table 3 The assembly of 1,2,3-triazoles from nitroolefin with NaN3
tion of DMF moieties greatly accelerated the nucleophilic sub-
stitution of NaN3 to 1a. The highest yield obtained by
HCP-DMF also highlighted that the polarity of the material is
pivotal even though it has poor pores. Thus, benzyl azide and
the reaction conditions as listed in entry 5 were viewed as
worthy of optimization for further study. Importantly, the
product can be isolated conveniently by simple filtration,
drying and concentration thanks to the complete conversion
of benzyl bromide and excellent selectivity to product.
The substrate scope of substituted benzyl bromides was
subsequently investigated under standard conditions, and the
results are given in Scheme 2. Diverse benzyl bromides
bearing diverse substituents were compatible with the stan-
dard conditions. Various substituents in the aryl ring of 1 are
tolerable, and the desired products 2b–2r can be obtained in
yields ranging from 88–95% without electronic and steric
effects. Compared to the above results, only inferior yields (2b
and 2f) were reached without the addition of HCP-DMF. It was
noted that 93% 2q was isolated when 1,2-dibenzyl bromide
and 2.3 equivalents of NaN3 were used as the starting
materials. A substrate containing N-heterocyclic ring 1r also
afforded a 95% yield of 2r (Scheme 2).
Entry
Solvent
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
Anisole
HCP-DMF-SO3H
HCP-DMF-SO3H
HCP-DMF-SO3H
HCP-DMF-SO3H
HCP-DMF-SO3H
HCP-Ben/DMF-SO3H
HCP-Ben-SO3H
Amberlyst 15
NR
NR
NR
24
46
32
21
17
79
92
iPrOAc
2-MeTHF
EtOH
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
9b
10b,c
HCP-DMF-SO3H
HCP-DMF-SO3H
a 3a, 0.3 mmol, NaN3: 0.35 mmol, catalyst: 10 mg, isolated yields.
b 20 mg catalyst. c 4 h.
meric matrix. However, with the commercial bead Amberlyst
15, 4a was obtained in 17% yield (entry 8), which was lower
than that of HCP-Ben-SO3H (21%). It is inferred that diffusion
limitation and less swelling of the bead Amberlyst 15 caused
its lower activity compared to powder HCP-Ben-SO3H. Finally,
the yield of 4a was enhanced to 92% by increasing the dosage
of HCP-DMF-SO3H to 20 mg and lengthening the reaction
time to 4 h (entries 9 and 10). The encouraging results proved
that the combination of HCP-DMF-SO3H with EtOH (95%)
indeed gave a suitable alternative to replace DMF in the syn-
thesis of 1,2,3-triazole from nitroolefin and NaN3.
After the optimized conditions were established based on
entry 10 in Table 3, we then investigated the reaction scope in
terms of nitroolefins (Scheme 3). Nitroolefins bearing electron-
donating or -withdrawing groups on aryl rings gave the corres-
ponding products in high to excellent yields (82%–94%)
Given the outstanding performance of HCP-DMF in EtOH
(95%), we would like to use its corresponding acidified
material HCP-DMF-SO3H as a catalyst to promote the 1,3-
dipolar cycloaddition of nitroolefin to NaN3 to synthesize
1,2,3-triazole which is a significant scaffold in widespread
drug analogs with bioactivities such as antimicrobial, antiviral
and antitumor effects.33 There are similar issues in that the
process is usually carried out in DMF or DMSO solvents and
thus is associated with environmental and health
concerns.7,45–47 The upgraded catalyst HCP-DMF-SO3H with a
pseudo-DMF microenvironment for acid sites gives us an
opportunity to replace DMF in this reaction. Some green sol-
vents were investigated again in the presence of 10 mg
HCP-DMF-SO3H and no conversion of 3a was observed in
i
anisole, PrOAc or 2-MeTHF at 60 °C (Table 3, entries 1–3).
What is promising is that a 24% yield of 4a was separated in
EtOH (entry 4), which improved to 46% when EtOH (95%) was
used (entry 5). The lower yields in entries 6 and 7 were prob-
ably caused by the decrease of M1 content in the catalyst poly-
Scheme 2 The substrate scope of benzyl bromides to benzyl azides.
a0.7 mmol NaN3 was added, the yields in parentheses were obtained
without the addition of HCP-DMF.
Scheme 3 Nitroolefin substrates to synthesize 1,2,3-triazoles. a10 h.
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within 4–6 h, suggesting that this polar catalyst manifested
great activity and selectivity toward diverse nitroolefins in
EtOH (95%) and the electronic nature of nitroolefins has only
a little influence on the 1,3-dipolar cycloaddition. In addition,
the reaction with a heterocyclic substituted nitroolefin, 3l, also
proceeded smoothly, giving the expected product 4l in 85%
yield. However, (E)-3-(2-nitrovinyl)-indole 3m was reluctant to
participate in the reaction and the target product 4m was only
obtained in 58%, probably resulting from the alkalinity of 3m.
The low activity of 3m was also observed in other reports so
harsh conditions were required to improve the yield of 4m.7
It is also possible to combine the conversion of benzyl
bromide to benzyl azide and the cycloaddition with nitroole-
fins to 1,4-disubstituted-1,2,3-triazoles together to establish a
tandem process. Out of this consideration, a three-component
reaction of 1a, 3a and NaN3 was investigated by using the
DMF-containing HCPs as promoting materials. As shown in
Scheme 4, 5a was obtained in 58% yield in the presence of
HCP-DMF-SO3H. When Amberlyst 15 was used as the catalyst,
the yield only reached 33%, indicating again the promoting
effect of the DMF moiety on the HCP-based catalyst
(Scheme 4).
Fig. 2 (a) Reaction kinetics for the synthesis of 4a from 3a and NaN3.
(b) Recycle experiment of HCP-DMF-SO3H in the 1,3-dipolar cyclo-
addition in EtOH (95%).
to the first run (92%) (Fig. 2). The maintained activity of
HCP-DMF-SO3H is probably explained in terms of the negli-
gible solubility of the sodium salt and the inertia of the hetero-
geneous acid catalyst in EtOH (95%). It should be noted that
the accumulation of sodium salt and insoluble substances
generated in the transformation may suppress the reaction
rate. For this reason, a longer time than 6 h is needed to
convert the reactants completely at the 7th recycle.
The reaction kinetics of the 1,3-dipolar cycloaddition of
nitroolefin to NaN3 further confirmed that the dipolar
HCP-DMF-SO3H catalyst was indeed beneficial for the syn-
thesis of 4a (Fig. 2a). It was noticed that the reaction rate of
HCP-Ben-SO3H was obviously faster than that of
HCP-DMF-SO3H at the initial stage before 2 h mainly because
of the large SBET of HCP-Ben-SO3H providing more active sites.
However, the yield of 4a in HCP-Ben-SO3H was distinctly
restrained after 2 hours resulting from the formation of by-pro-
ducts. By contrast, the reaction rate of HCP-DMF-SO3H
remained constant until the substrate was gradually consumed
due to its solvation to NaN3 and the higher selectivity to target
product 4a.
The recycling of these two HCP materials was also investi-
gated. HCP-DMF can be efficiently recovered and reused in the
model reaction. However, for the recycling of HCP-DMF-SO3H,
we have to consider the possible damage of its activity because
H+ might be exchanged by the Na+ of the generated byproduct,
a sodium salt, in the cycloaddition. The reuse performance of
HCP-DMF-SO3H was investigated carefully in the reaction of
β-nitrostyrene and NaN3 under standard conditions.
Interestingly, the polar catalysts remained highly active even
after 7 successive recycles, supporting excellent yields of pro-
ducts above 84% without a dramatic yield decrease compared
Conclusions
Two
HCP-based
polar
materials,
HCP-DMF
and
HCP-DMF-SO3H, were prepared by hypercrosslinking of DMF-
containing monomers. The creation of a DMF-like microenvi-
ronment and the enhanced polarity of HCP-DMF was disclosed
by FT-IR and fluorescence spectra. These two HCP-based
materials exhibited quite a good performance in assisting the
conversion of NaN3 to benzylic azides and the synthesis of
1,2,3-triazoles. In all the reactions, ethanol was used as the
solvent, avoiding the utilization of hazardous DMF solvent.
The polar HCP-based materials can be easily recycled as well.
The DMF moieties in these two materials played a crucial role
in ensuring a good performance in the model reaction. On the
basis of this work, it is conceivable that the task-specific dec-
oration of heterogenous catalysts with accessible solvation
microenvironments and high surface areas may be a new facet
of green catalysis and synthesis. This type of topic for material
design can contribute not only to eliminating the use of hazar-
dous solvents but also to enhancing catalytic performance
through synergistic catalytic effect.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The National Natural Science Foundation of China (No.
21872060, 21902054, 21761132014 and 22072049) are grate-
Scheme 4 The tandem reaction of 1a, 3a and NaN3 to synthesize 1,4-
disubstituted-1,2,3-triazoles 5a with the assistance of HCP-DMF-SO3H.
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fully acknowledged for financial support. This work was also
supported by The Innovation and Talent Recruitment Base of
New Energy Chemistry and Device.
F. Albericio, F. Dettner, C. C. Lechner, R. Schönleber and
D. S. Pedersen, Green Chem., 2021, 23, 3312–3321.
19 A. Kumar, M. Alhassan, J. Lopez, F. Albericio and B. G. de
la Torre, ChemSusChem, 2020, 13, 5288–5294.
20 X. Guo, J. Tang, B. Xiang, L. Zhu, H. Yang and C. Hu,
ChemCatChem, 2017, 9, 3218–3225.
21 Q. Sun, S. Wang, B. Aguila, X. Meng, S. Ma and F.-S. Xiao,
Nat. Commun., 2018, 9, 3236–3243.
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