Design and Synthesis of Zirconium-Containing Coordination Polymer Based on Unsymmetric Indolyl Dicarboxylic Acid and Catalytic Application on Borrowing Hydrogen Reaction

Article abstract of DOI:10.1002/adsc.201800875

Catalytic borrowing hydrogen reaction is a very attractive transformation in the field of C-alkylation reaction. In this work, a new Zr (Zirconium)-containing coordination polymer containing unsymmetric indolyl dicarboxylic acid 1-(carboxymethyl)-1H-indole-5-carboxylic acid (H₂CIA) was synthesized by the way of a solvothermal synthetic route and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Nitrogen adsorption-desorption, fourier transform infrared spectroscopy and X-ray photoelectronic spectroscopy (XPS). The coordination polymer Zr-CIA was employed as the catalyst for C-alkylation of acetophenone derivatives in the presence of benzyl alcohol. In addition, Zr-CIA catalyst was also observed to be effective in the reaction of alcohols with alcohols and high yields of alkylation products were achieved. Mechanism investigations were also conducted to better understand the catalysts and transformations. Meanwhile, the Zr-CIA could be reused at least five times without a notable decrease in activity and selectivity. (Figure presented.).

Full text of DOI:10.1002/adsc.201800875

Title: Design and Synthesis of Zirconium-Containing Coordination
Polymer Based on Unsymmetric Indolyl Dicarboxylic Acid and
Catalytic Application on Borrowing Hydrogen Reaction
Authors: Xinyu Hu, Haiyan Zhu, Xinxin Sang, and Dawei Wang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800875
Link to VoR: http://dx.doi.org/10.1002/adsc.201800875

10.1002/adsc.201800875
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.200((will be filled in by the editorial staff))
Design and Synthesis of Zirconium-Containing Coordination Polymer Based on
Unsymmetric Indolyl Dicarboxylic Acid and Catalytic Application on
Borrowing Hydrogen Reaction
Xinyu Hu,a Haiyan Zhu,a Xinxin Sang,*a Dawei Wang*a,b
a
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material
Engineering, Jiangnan University, Wuxi 214122, Jiangsu Province, China. E-mail: sangxx@jiangnan.edu.cn,
wangdw@jiangnan.edu.cn
Key laboratory of inorganic nonmetallic crystalline and energy conversion materials, College of Materials and
b
Chemical Engineering, China Three Gorges University, Yichang, Hubei 443002, China.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200######. ((Please delete if not appropriate))
supported catalysts might be reduced due to the leaching
problem for borrowing hydrogen reaction. Therefore, it is
of great importance to develop heterogeneous catalysis in
place of classical supported catalysts for C-alkylation
reaction, especially, inexpensive and easy preparation, high
activity and outstanding selectivity of the recovered
catalysts.
Nowadays, a great deal of coordination polymers based
on multitopic organic spacers and metal ions have been
self-assembled.[14] The structures and properties of
polymers mainly depended on the natures of organic
bridging ligands and metal centres. Coordination polymers
with fascinating structures and new topologies have
potential applications in catalysis, medicine and materials
science.[15] Because of abundant conformations and
coordination topics, many coordination polymers based on
carboxylic acids have been reported.[16] Most of these
Abstract: Catalytic borrowing hydrogen reaction is a very
attractive transformation in the field of C-alkylation
reaction. In this work, a new Zr (Zirconium)-containing
coordination polymer containing unsymmetric indolyl
dicarboxylic
acid
1-(carboxymethyl)-1H-indole-5-
carboxylic acid (H2CIA) was synthesized by the way of a
solvothermal synthetic route and characterized by powder
X-ray diffraction (XRD), scanning electron microscopy
(SEM), transmission electron microscopy (TEM), Nitrogen
adsorption-desorption,
fourier
transform
infrared
spectroscopy and X-ray photoelectronic spectroscopy
(XPS). The coordination polymer Zr-CIA was employed as
the catalyst for C-alkylation of acetophenone derivatives in
the presence of benzyl alcohol. In addition, Zr-CIA catalyst
was also observed to be effective in the reaction of alcohols
with alcohols and high yields of alkylation products were
achieved. Mechanism investigations were also conducted to
better understand the catalysts and transformations.
Meanwhile, the Zr-CIA could be reused at least five times
without a notable decrease in activity and selectivity.
carboxylate ligands possessed
a
highly symmetric
geometry which is beneficial for symmetric networks
formation. In contrast, coordination polymers based on
asymmetric carboxylate ligands are far less prevalent,
probably due to the difficulty to predict the final
coordination networks with unsymmetric geometry.
Compared to the symmetric ligands, the unsymmetric
ligands have different conformations and coordination
modes, which could effectively avoid framework
interpenetrations and further construct complexes with
great structural diversity.[17] The unsymmetry of organic
ligands is likely to lead to structural defects, which made
positive contribution to the catalytic performance. As far as
we know, the preparation of organic-inorganic hybrid
catalysts using unsymmetric carboxylic ligands as a
building block has not been reported. Based on our
previous work on unsymmetric ligands[18] and borrowing
hydrogen reaction,[19] herein, we designed a new porous
zirconium unsymmetric carboxylic complex based on
indolyl dicarboxylic acid skeleton 1-(carboxymethyl)-1H-
indole-5-carboxylic acid (H2CIA), denoted as Zr-CIA,
which could be applied as a highly efficient heterogeneous
Keywords: zirconium, polymers, catalysis, borrowing
hydrogen, unsymmetry
C-alkylation of ketones with alcohols as alkylating
agents, through a borrowing hydrogen (BH) or hydrogen
autotransfer (HA) strategy is of great importance due to its
wide application in material science and pharmaceutical
industry.[1] Transition-metal-catalyzed -alkylation of
ketones through a borrowing hydrogen pathway has
aroused considerable attention for constructing C-C bond.
Homogeneous noble metal catalysts such as Ru[2] and Ir[3]
complexes were known to be effective catalysts for this
reaction. Other metal complexes such as Rh,[4] Au,[5] Cu,[6]
Fe,[7] Pd[8] and Os[9] were also reported as the alternative
catalysts.[10-13] Due to catalyst-product separation and
recycle issues, heterogeneous catalysts were developed to
as much better and effective methods, such as supported
catalysts. Moreover, the stability and reusability of the
1
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10.1002/adsc.201800875
Advanced Synthesis & Catalysis
catalyst for C-alkylation of ketones with alcohols, and
alcohols with alcohols through borrowing-hydrogen
strategy.
broadening of diffraction peaks indicated the formation of
small Zr-CIA nano-crystallites. Furthermore, X-ray
photoelectron
spectroscopy
(XPS)
showed
the
characteristic peaks of Zr3d, O1s and N1s (Figure 1e),
suggesting Zr successfully coordinated with Zr-CIA.
Thermogravimetric analysis (TGA) under N2 atmosphere
Catalyst preparation and characterization
In the beginning, this unsymmetric carboxylic ligand
was synthesized according to our similar experimental
steps in gold catalyzed allene synthesis.[20] As illustrated in
Scheme 1, methyl indole-5-carboxylate (1a), methyl 2-
bromoacetate, anhydrous potassium carbonate were mixed
in acetonitrile (MeCN). After twenty hours at refluxed
conditions, the pale yellow diester (1b) was achieved in
88 % yield. Next, diester (1b) was dissolved in the mixed
solvent (THF: H2O = 1:1), and LiOH.H2O (1 M) was added
to the mixture at room temperature. After stirring eight
hours, the mixture was acidified to pH 2, the product 1-
(carboxymethyl)-1H-indole-5-carboxylic acid (H2CIA) (1c)
was obtained in 95 % yield. The structure of H2CIA was
confirmed by 1H NMR, 13C NMR and HRMS. The detailed
synthesis steps were added in the supporting information.
o
revealed that the Zr-CIA kept stable up to 400 C (Figure
1f), guaranteeing the stability of Zr-CIA used for catalytic
performance. The porosity of Zr-CIA was determined by
N2 adsorption-desorption isotherm (Figure S1), and the
Brunauer-Emmett-Teller (BET) surface area, pore volume
were 23.8 m2 g-1, 0.258 cm3 g-1 respectively. The low
surface area of Zr-CIA maybe caused by the use of DMF
as the solvent. From N2 adsorption-desorption isotherm in
Figure S1, N2 adsorption capacity was negligible when the
relative pressure (P/P0) was as low as 0.01, illustrating
little micropores in Zr-CIA.
Scheme 1. The synthesis of unsymmetric indolyl
dicarboxylic acid ligand.
Subsequently, the coordination polymer Zr-CIA was
synthesized by the solvothermal synthetic route of 1-
(carboxymethyl)-1H-indole-5-carboxylic acid (H2CIA)
with ZrCl4 (The detailed steps were added in the
supporting information). The prepared Zr-CIA polymer
was characterized by fourier transform infrared
spectroscopy (FT-IR), X-ray diffraction (XRD), scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM). The FT-IR spectrum of the as-
prepared catalyst displayed the characteristic unsymmetric
(1580 cm-1) and symmetric vibrations (1395 cm-1) of
carboxylate groups in deprotonated CIA (H2CIA that lost
hydrogen), which were narrowed compared with the
spectrum of CIA (Figure 1a). This indicated that the
carboxylate groups of CIA were coordinated to Zr4+ ions.
Despite huge efforts had been made to synthesize Zr-CIA
single crystal, only nanoparticals were obtained probably
Figure 1. Characterization of Zr-CIA. (a) Powder XRD
pattern, (b) FT-IR spectra, (c) SEM image, (d) TEM image,
(e) XPS and (f) TGA.
Catalytic activity
In the initial investigation, C-alkylation of acetophenone
with benzyl alcohol was selected as the model reaction to
explore the influence of various parameters. The results
were summarized in Table 1. The reaction did not proceed
without the catalysts (entries 1-3). Inorganic salts
containing Zr revealed no catalytic activity under the same
condition (entries 4-7). Commercial ZrO2 also showed low
activity with a yield less than 10% (entry 8). Surprisingly,
the reaction proceeded with much higher yield when the
as-prepared Zr-CIA was used as the catalyst in toluene
(entry 9). The variety of bases was investigated (entries 9-
12) and the results indicated that Zr-CIA together with
both Cs2CO3 or NaOH can greatly accelerate the reaction.
Furthermore, the solvent effect of this reaction was studied
(entries 12-22) and toluene was proved to be the most
suitable solvent for this reaction.
due to the fast formation of strong Zr-O bonds. The XRD
o
pattern showed only one diffraction peak at 6.5
,
suggesting that the obtained Zr-CIA was partially
crystallized or ordered (Figure 1b). In the structure of CIA,
the asymmetric coordination groups (carboxylate) resulted
in a lower degree of crystallinity for the as-prepared
catalyst. It was noted that the X-ray diffraction was
broadened, suggesting that the Zr-CIA was composed of
small nano-crystallites. SEM and TEM examinations
demonstrated that Zr-CIA was spherical shaped with an
average size of 100 nm (Figure 1c and 1d). The SEM and
TEM results were consistent with that of XRD, i.e. the
2
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10.1002/adsc.201800875
Advanced Synthesis & Catalysis
Table 1. Optimization of reaction conditions [a,b]
However, the isolated yields based on para-halogenated
acetophenone were lower than 4'-methylacetophenone.
Compared with acetophenones possessing electron-
donating groups, electron-poor ones gave the
corresponding products in lower yields (Table 2, 4o and
4p).
When
3'-methoxyacetophenone
and
4'-
Entry
1
2
3
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Catalyst
Base
-
-
Cs2CO3
Cs2CO3
Cs2CO3
Solvent
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
Xylene
Dioxane
DMF
DCM
DCE
EtOAc
EtOH
CH3CN
Benzene
Yield[%]
<1
<10
<10
<10
<10
<5
<10
85
10
methoxyacetophenone were utilized as substrates, the
desired ketone products 4r-4z were isolated with good to
excellent yields whether benzylic alcohols bearing
electron-withdrawing or donating groups were used. In
addition, 2-acetylnaphthalene was found to be less active
for this reaction contrasted with methoxyacetophenones,
and 4aa was isolated only in 80% yield.
-
Zr-CIA
-
.
ZrOCl2 8H2O
.
Zr(SO4)2 4H2O
ZrO(NO3)2.xH2O Cs2CO3
ZrO2
Cs2CO3
Cs2CO3
K2CO3
NEt3
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Zr-CIA
Table 2. Substrate expansion of the acetophenone
<5
80
<5
10
10
<5
<5
<5
5
5
<5
64
derivatives with alcohols. [a,b]
NaOH
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
22
[a]
Reagents and conditions: 2a (1.0 mmol), 3a (2.0 mmol), [M]
(4.0 % mmol), base (2.0 mmol), solvent (3 mL), reflux, 12 h.
[a] Isolated yield.
Catalytic performance of Zr-CIA for the reaction of
acetophenone derivatives with alcohols.
To explore the applicability of Zr-CIA catalyst for a
broader range of substrates, we next employed the
optimized reaction conditions (entry 9, Table 1) for the
coupling between a variety of alcohols and ketones. Firstly,
we investigated the substrate scope of ketones while using
benzyl alcohol as the alkylating reagent. 4'-
methoxyacetophenone was found to be an excellent
substrate for this reaction, and 4d was isolated in 92%
yield. Whereas acetophenone containing vicinal methyl
group on the benzene ring was a less effective substrate
(75%, 4g), 3'-methylacetophenone, 4'-methylacetophenone
and 4'-ethylacetophenone reacted smoothly with benzyl
alcohol to provide the desired products in 80%, 86% and
80% yield, respectively.
Next, a series of aromatic primary alcohols were
chosen to react with acetophenone, and the results were
summarized in Table 2. Differently substituted benzylic
alcohols were suitable substrates for the indicated reactions
under the standard conditions, and in most cases, using
benzylic alcohols bearing electron-withdrawing or
donating groups, the desired ketone products 4i-4l were
isolated with good to excellent yields. When 2-
methoxybenzyl alcohol was utilized as a substrate, the
conversion of 2'-methylacetophenone was lower than 3'-
methylacetophenone and 4'-methylacetophenone (4m, 4n,
4q). 3'-methylacetophenone and 4'-methylacetophenone
reacted smoothly with 2-methoxybenzyl alcohol to provide
the desired products in 89% and 93% yield respectively.
[a] Conditions: 2 (1.0 mmol), 3 (2.0 mmol), Zr-CIA (4.0 % mmol),
Cs2CO3 (2.0 mmol), toluene (3 mL), 12 h, 110 oC.
[b] Isolated yields based on 2.
Catalytic performance of Zr-CIA for the reaction of
alcohols with alcohols.
The borrowing hydrogen transformation with respect to
alcohols and alcohols was much more interesting than that
of ketones and alcohols. Meanwhile, it was considered that
it was a quite difficult problem to scientists in this area. It
was famous that this reaction underwent one key step: the
successive dehydrogenation of alcohols to ketones or
3
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10.1002/adsc.201800875
Advanced Synthesis & Catalysis
aldehydes. The reaction pathway was shown in Scheme
2.[21] Among this transformation, high activity catalysts
played the key role and determined the possible pathway
and reaction efficiency.
This excellent catalytic activity of Zr-CIA polymer
system in the reaction of alcohols and alcohols inspired us
to explore the possible mechanism to better cognize this
Zr-CIA polymer. Importantly, the blank experiment
indicated that no borrowing hydrogen product was
obtained, when simple zirconium salts were examined to
this transformation. It should be noted that high catalytic
activity was achieved, when the unsymmetrical indolyl
dicarboxylic acid functional group was introduced to this
catalytic system. It has been deduced that the higher
acidity and a more open framework with large number of
open sites affected the activity of the catalysts for the
borrowing hydrogen reaction of carbonyl compounds. As
shown in Table 1, Zr-CIA showed a much better
performance than the common catalyst ZrO2, which might
be caused by the higher lewis acidity and lower
crystallinity of Zr-CIA.[22] It was observed that Zr element
was crucial for the borrowing hydrogen reaction. For better
explaining this, we examined the local environment of Zr
species in Zr-CIA and ZrO2 by XPS. As shown in Figure
2A, the binding energies of both Zr 3d5/2 (182.6 eV) and Zr
3d3/2 (185.0 eV) in Zr-CIA were higher than in ZrO2 (181.8
eV for Zr 3d5/2 and 184.2 eV for Zr 3d3/2). The higher
binding energy of Zr 3d in Zr-CIA indicated a higher
positive charge on the Zr atoms, which resulted in a
stronger Lewis acidity of Zr. The higher Lewis acidity of
Zr in Zr-CIA was beneficial to activating the carbonyl
groups and thus increased the total reaction rate. The local
environment of O elements in Zr-CIA and ZrO2 were also
examined (Figure 2B). The peak at 529.8 eV is
corresponding to lattice oxygen and 531.7 eV is C(O)OH.
Obviously, lattice oxygen in Zr-CIA was much less than
ZrO2. This can be attributed to the unsymmetrical ligand
using this strategy against coordination and crystallization,
resulting in a type of structural “imperfection” and large
number of open sites. The fact of structural imperfection
boosted the catalytic activity of the coordination polymers.
Therefore, the lower crystallinity and the presence of
C(O)OH in Zr-CIA were in favour of improving Zr
catalytic activity. Furthermore small nano-crystallites in
the material help for higher reactivity and selectivity due to
its partially ordered structure and the presence of surface
[Zr4+]lattice with lower coordination numbers.
Scheme 2. The borrowing hydrogen reaction of primary
and secondary alcohols.
Encouraged by these significative results, we further
employed this Zr-CIA catalyst to the transformation of
alcohols and alcohols. We were pleased to find that the
reaction of alcohols and alcohols was realized with this
method and the results were summarized in Table 3. The
substituent groups of these alcohols had a minimal
influence to this transformation and yields of the
corresponding products were isolated in 69 % to 81 %. It
should be noted that the reaction didn’t happen only
with H2CIA, no base and solvent-free conditions.
Table 3. Substrate expansion of the alcohols with alcohols.
[a,b]
[a] Conditions: 5 (1.0 mmol), 3 (2.0 mmol), Zr-CIA (4.0 % mmol),
Cs2CO3 (2.0 mmol), toluene (3 mL), 18 h, 110 oC.
[b]Isolated yields based on 5.
Figure 2. XPS spectra of (A) Zr 3d and (B) O 1s in (a)
ZrO2 and (b) Zr-CIA
[c] only H2CIA, no base, solvent-free.
4
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10.1002/adsc.201800875
Advanced Synthesis & Catalysis
To further understand this possible mechanism of the
reaction pathway. So, the competition experiments for
radical progress were carried out with one equivalent of
TEMPO (Scheme 4). The results showed that the
corresponding products were isolated in nearly the same
yield, which ruled out the possibility of free-radical
progress during these transformations.
reaction between primary alcohols and secondary alcohols,
the capture of key intermediates would be an effective and
meaningful method. We were glad to find that intermediate
7a was isolated in 38% yield when the substrate (3a) was
treated with Zr-CIA coordination polymer as a catalyst.
Moreover, up to 92% yield of product (6a) was obtained
when the substrate (5a) was dealt under the upper
conditions, which was supported by the previous results
(Scheme 3).
Scheme 4. Control experiments with radical scavenger.
The reusability of Zr-CIA was examined through the
borrowing hydrogen reaction of acetophenone and benzyl
alcohol under the upper optimum conditions. The catalyst
Zr-CIA could be separated conveniently by high speed
centrifugation, washed with EtOH, deionized water and
then vacuum-drying for circulation. Next, the recovered
catalyst Zr-CIA was used for the next cycle. It was found
that Zr-CIA could be reused at least five cycles without
reducing the conversion of acetophenone and the yield of
4a, as is shown in Figure 3. After being used for five times,
the conversion and selectivity were up to 95 % and 80 %,
respectively.
Scheme 3. The separation of reaction intermediates.
Meanwhile, all the ketone intermediates synthesis
experiments were carried out to better illustrate the catalyst
and the results were concluded in Table 4. As be expected,
almost all the alcohols were converted into ketone products
smoothly. It was found that the alcohol substrates with
functional groups such as Cl, CH3O, F, and CH3 could be
preferable tolerated under the optimized conditions, the
corresponding reduction products were isolated in high to
excellent yields.
Table 4. The synthesis of all the ketones intermediates. [a,b]
Figure 3. Reuse of Zr-CIA catalyst in borrowing hydrogen
reaction of acetophenone with benzyl alcohol.
Finally, the recovered Zr-CIA was characterized by FT-
IR and SEM (Figure 4). The experiments revealed that the
properties of Zr-CIA did not change notably after being
reused five times, indicating the excellent stability of Zr-
CIA as heterogeneous catalyst in borrowing hydrogen
reaction.
[a]
Conditions: 5 (1.0 mmol), Zr-CIA (4.0 % mmol), Cs2CO3 (2.0
mmol), toluene (3 mL), 18 h, 110 oC.
[b] Isolated yields based on 5.
In
addition,
2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) was used as radical scavenger to examine
5
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10.1002/adsc.201800875
Advanced Synthesis & Catalysis
Central Universities (JUSRP 51627B) and MOE & SAFEA
for the 111 Project (B13025).
Figure 4. The FT-IR spectrum (a) and SEM image (b) of
the recycled Zr-CIA.
In summary, we have designed a porous Zr-CIA
References
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a
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asymmetric acid 5-carboxyl indol-1-yl-acetic acid (H2CIA)
as the starting material. The obtained Zr-CIA coordination
polymer was successfully applied to catalyze α-alkylation
of ketones with primary alcohols through a hydrogen
borrowing strategy in high yields. Furthermore, the Zr-CIA
catalyst was also proved to be effective for the C-C bond-
forming between a variety of alcohols and a range of
alcohols as alkylating reagents. The catalyst was reused at
least five times without reducing the activity and
selectivity. The present methodology further expanded the
applications of zirconium catalysts in synthetic chemistry
regarding the C-C bond-forming process.
Experimental Section
General procedure for the alkylation of ketones with
alcohols
To 20 mL Schlenk tube was added Zr-CIA (4.0% mmol),
toluene (3 mL), alcohols (2.0 mmol), ketones (1.0 mmol)
and caesium carbonate (2.0 mmol). The mixture was
heated and stirred under 110 oC for 12 h and then cooled to
room temperature. The resulting solution was directly
purified by column chromatography with petroleum
ether/ethyl acetate (v/v = 40:1) as eluent to give the desired
product.
General procedure for cross-coupling of secondary and
primary alcohols
To 20 mL Schlenk tube was added Zr-CIA (4.0 % mmol),
toluene (3.0 mL), Primary alcohol (1.0 mmol), Secondary
alcohol (2.0 mmol) and caesium carbonate (2.0 mmol).
o
The mixture was heated and stirred under 110 C for 18 h
and then cooled to room temperature. The resulting
solution was directly purified by column chromatography
with petroleum ether/ethyl acetate (v/v = 40:1) as eluent to
give the desired product.
Representative procedure for the preparation of
ketones
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work by the National Natural Science Foundation of China
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Advanced Synthesis & Catalysis
COMMUNICATION
Design and Synthesis of Zirconium-
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Adv. Synth. Catal. Year, Volume, Page – Page
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Wang*a
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