Polymer supported N-heterocyclic carbene ruthenium complex catalyzed transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.catcom.2019.105924

Polymer supported N-heterocyclic carbene ruthenium complex (Poly-Ru-1) was synthesized by free radical polymerization of Ru complex monomer and divinylbenzene. The structure of Poly-Ru-1 was characterized by ¹³C solid-state NMR spectroscopy, N₂ adsorption-desorption, X-ray photoelectron spectroscopy (XPS). ¹³C solid-state NMR spectra indicated the complete polymerization of Ru complex monomer. Poly-Ru-1 had the high specific surface area of 286.7 m² g^?1 and ruthenium was in +2 oxidation state. Poly-Ru-1 showed higher catalytic activity than the corresponding homogenous catalyst in transfer hydrogenation of ketones, which was attributed to the high dispersion of the active sites.

Full text of DOI:10.1016/j.catcom.2019.105924

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Polymer supported N-heterocyclic carbene ruthenium complex
catalyzed transfer hydrogenation of ketones
Rong Wang, Li Qin, Xin Wang, Bihua Chen, Yun Zhao, Guohua
Gao
PII:
S1566-7367(19)30385-1
DOI:
https://doi.org/10.1016/j.catcom.2019.105924
CATCOM 105924
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 September 2019
18 December 2019
31 December 2019
Please cite this article as: R. Wang, L. Qin, X. Wang, et al., Polymer supported N-
heterocyclic carbene ruthenium complex catalyzed transfer hydrogenation of ketones,
Catalysis Communications (2019), https://doi.org/10.1016/j.catcom.2019.105924
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© 2019 Published by Elsevier.

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Polymer supported N-heterocyclic carbene ruthenium complex catalyzed
transfer hydrogenation of ketones
Rong Wang, Li Qin, Xin Wang, Bihua Chen, Yun Zhao, Guohua Gao*
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, 3663 North
Zhongshan Road, Shanghai 200062, China
*Corresponding author: Guohua Gao. Tel. and Fax: +86-021-62233323. E-mail:
ghgao@chem.ecnu.edu.cn
Abstract
Polymer supported N-heterocyclic carbene ruthenium complex (Poly-Ru-1) was
synthesized by free radical polymerization of Ru complex monomer and
divinylbenzene. The structure of Poly-Ru-1 was characterized by 13C solid-state NMR
spectroscopy, N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS). 13C
solid-state NMR spectra indicated the complete polymerization of Ru complex
monomer. Poly-Ru-1 had the high specific surface area of 286.7 m2 g-1 and ruthenium
was in +2 oxidation state. Poly-Ru-1 showed higher catalytic activity than the
corresponding homogenous catalyst in transfer hydrogenation of ketones, which was
attributed to the high dispersion of the active sites.
Keywords: polymer; support; ruthenium complex; transfer hydrogenation.
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1. Introduction
As one of the reactions to produce valuable alcohols, the reaction of transfer
hydrogenation of ketones was widely applied in pharmaceuticals, fuels, building
blocks and synthetic intermediates in organic synthesis [1-3]. In the transfer
hydrogenation reaction, 2-propanol was generally used as hydrogen donor instead of
conventional hydrogen gas which usually involved the elaborate experiment setups
like high temperature and pressure reactors [4-6]. Transition metal complexes were
used to catalyze the transfer hydrogenation of acetophenone. Compared with rhodium
and iridium complexes, ruthenium complexes were more suitable for the reaction by
virtue of their high activity and low cost [7-9]. The homogeneous ruthenium complex
catalysts usually suffered from the difficulty of separation and recycle [10]. To solve
these problems, heterogeneous catalysts prepared by immobilizing ruthenium
complexes on a variety of solid materials, including organic polymers, silicon and
magnetic materials were applied in the transfer hydrogenation reaction [11-14].
Nevertheless, they usually demonstrated poorer activities compared with
homogeneous catalysts due to the lower degree of exposure of active sites to reactants
[14, 15]. Therefore, it is still a challenge to develop new and efficient activity
heterogeneous catalysts.
Due to the tunability of ionic liquid structure, functional ionic liquid can be
designed according to requirements. The ionic liquids containing vinyl groups have
the potential of preparing polymers. Poly(ionic liquid)s (PILs) had been applied as
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catalysts and showed outstanding catalytic activity in cycloaddition reaction of CO2,
hydrolysis of ethylene carbonates and transesterification reaction [16-20]. Besides,
vinyl imidazolium ionic liquids can act as N-heterocyclic carbene (NHC) ligand to
coordinate with metal complexes. Through heterogenization of this kind of
homogeneous metal complex, the polymer supported metal was prepared and could be
utilized in many catalytic reactions [21, 22].
In this paper, the imidazolium ionic liquid containing polymerizable vinyl group
was chosen to be the ligand for the metal complex. Polymer supported ruthenium
complex, Poly-Ru-1, was prepared through the free radical polymerization of Ru
complex monomer and divinylbenzene (DVB). Poly-Ru-1 showed higher activity than
the corresponding homogeneous Ru complex monomer in the transfer hydrogenation
of acetophenone.
2. Results and discussion
2.1 Synthesis of Poly-Ru-1 and Poly-Ru-2.
The synthetic procedure of catalysts Poly-Ru-1 and Poly-Ru-2 was shown in
Scheme 1. Ru complex monomer containing polymerizable vinyl group was obtained
through the transmetallization of silver complex using [Ru(p-cymene)Cl2]2.
Poly-Ru-1 was prepared by the free radical polymerization of Ru complex monomer
and DVB using 2,2'-azoisobutyronitrile (AIBN) as an initiator in refluxing chloroform
for 24 h. Similarly, Poly-Ru-2 was obtained by the free radical polymerization of Ru
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complex monomer without any cross-linker. Poly(ionic liquid) (PIL-1) was prepared
by free radical polymerization of 1, 3-bis (4-styryl-1-yl) imidazolium chloride and
DVB (Scheme S1).
Scheme 1. Synthetic procedure of Poly-Ru-1 and Poly-Ru-2.
2.2 Characterizations of Poly-Ru-1 and Poly-Ru-2
The solid-state 13C MAS NMR spectra of Poly-Ru-1, Poly-Ru-2, PIL-1, and the
13C NMR spectrum of corresponding Ru complex monomer were shown in Fig. 1.
The signals at 108 ppm that belonged to the N-vinyl group disappeared in Poly-Ru-1,
Poly-Ru-2 and PIL-1, indicating the full polymerization of the monomers. The signals
around 171 ppm in Poly-Ru-1 and Poly-Ru-2 were assigned to C2 atom of
imidazolium ring coordinating with Ru atom in comparison with PIL-1. The signals
around 82 ppm in Poly-Ru-1 and Poly-Ru-2 were attributed to the benzene ring of
p-methyl isopropyl benzene in Ru complex monomer (Fig. 1b, c). These results
indicated the Ru NHC complex was successfully included in the polymer. The signals
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around 40 ppm represented the carbon atoms in the polymer chains of PIL-1,
Poly-Ru-1 and Poly-Ru-2.
Fig. 1. (a) 13C NMR spectra of Ru complex monomer and solid-state 13C MAS NMR
spectra of (b) Poly-Ru-1, (c) Poly-Ru-2, (d) PIL-1.
Specific surface areas of Poly-Ru-1 and Poly-Ru-2 were obtained by the N2
adsorption-desorption isotherm. Poly-Ru-1 had a specific surface area up to 286.7 m2
g-1 (Table S1). The isotherm of Poly-Ru-1 displayed a clear type IV curve with an
obvious typical H3 hysteresis loop at the relative pressure P/P0 ranging from 0.35 to
0.98, which was the feature of mesoporous materials (Fig. 2). Similarly, Poly-Ru-2
had a specific surface area of 305.5 m2 g-1. The high surface area of Poly-Ru-1 and
Poly-Ru-2 were attributed to the rigid structure of Ru complex monomer. The slightly
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difference of surface area between the two samples might be caused by the higher
degree of rigid frameworks in Poly-Ru-2. The thermal stability of Poly-Ru-1 and
Poly-Ru-2 was measured by Thermo gravimetric analyzer (TGA) (Fig. S1).
o
Poly-Ru-1 and Poly-Ru-2 were stable up to 210 C. The chemical composition of
Poly-Ru-1 and Poly-Ru-2 was determined by element (CHN) analysis and the
contents of imidazolium according to nitrogen contents were 0.96 and 1.79 mmol g-1
respectively. The ruthenium contents of Poly-Ru-1 and Poly-Ru-2 measured by
inductive coupled plasma emission spectrometer (ICP) were 7.6 and 14.3 wt%,
respectively, which were close to the theoretical contents of two samples (Table S1).
The morphology of Poly-Ru-1 and Poly-Ru-2 was characterized by scanning electron
microscope (SEM), and SEM images showed that they were composed of irregular
solid blocks (Fig. S2). The TEM images of the fresh catalysts such as Poly-Ru-1 and
Poly-Ru-2 did not observed any ruthenium particles indicating the Ru complex was
stable during the polymerization (Fig. S3a and b).
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Fig. 2. N2 adsorption-desorption isotherms of Poly-Ru-1 and Poly-Ru-2.
The X-ray photoelectron spectroscopy (XPS) was employed to further
characterize the elemental composition and oxidation state of ruthenium in Poly-Ru-1.
XPS of Poly-Ru-1 showed the characteristic peaks of O 1s, N 1s, C 1s, Ru 3p, Ru 3d
and Cl 2s (Fig. 3a). The Ru 3d5/2 photoelectron peak in Poly-Ru-1 was observed at
281.6 eV, which was similar to the peak of [Ru(II)(p-cymene)Cl2]2 in Fig. 3b and the
Ru(II) complex of [RuCl(R)-(binap)(p-cymene)]Cl reported previously [23]. These
results suggested that the ruthenium was +2 oxidation state in Poly-Ru-1.
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Fig. 3. XPS of (a) full scale of Poly-Ru-1 and [Ru(p-cymene)Cl2]2, (b) Ru 3d region
of Poly-Ru-1 and [Ru (p-cymene)Cl2]2.
Table 2. Transfer hydrogenation of acetophenone catalyzed by various catalystsa.
Entry Cat.
Amount
of catb.
(mol %)
0.50
Base
Amount of Temp. Time
Yieldc
(%)
base
(oC)
(h)
(mol %)
1
2
3
Poly-Ru-1
Poly-Ru-2
KOtBu 10
KOtBu 10
KOtBu 10
82
82
82
6
91
32
19
0.50
6
6
Ru complex 0.50
monomer
4
5
6
7
8
Poly-Ru(0)
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
2.50
0.25
1.25
2.50
2.50
KOtBu 10
KOtBu 10
KOtBu 10
KOtBu 10
KOtBu 10
82
82
82
82
82
6
21
78
96
99
42
6
6
6
0.5
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9
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
Poly-Ru-1
2.50
2.50
2.50
2.50
2.50
2.50
2.50
2.50
2.50
2.50
KOtBu 10
KOtBu 10
KOtBu 10
KOtBu 10
KOtBu 10
82
82
82
40
60
82
82
82
82
82
1
51
86
95
11
42
7
10
11
12
13
14
15
16
17
18
2.5
4
6
6
-
-
6
KOtBu
KOtBu
KOH
6
6
83
90
95
9
8
6
10
6
Na2CO3 10
6
a Reaction conditions: acetophenone (0.24 g, 2 mmol), 2-propanol (6 mL).
b Base on the amount of Ru in catalyst.
c Determined by GC.
2.3 Transfer hydrogenation of ketones
2.3.1 Transfer hydrogenation of acetophenone catalyzed by various catalysts
Transfer hydrogenation of acetophenone and 2-propanol was carried out in the
presence of various ruthenium catalysts, including heterogeneous catalysts of
Poly-Ru-1, Poly-Ru-2 and corresponding homogeneous Ru complex monomer. The
results were summarized in Table 2. Poly-Ru-1 gave 1-phenylethyl alcohol in the
yield of 91% (Table 2, entry 1). However, when the Poly-Ru-2 was applied in
transfer hydrogenation, the yield dramatically decreased to 32% (Table 2, entry 2).
The homogeneous Ru complex monomer also gave a low yield of 19% (Table 2,
entry 3). Meanwhile, the polymer supported Ru nanoparticles (Poly-Ru(0)) which
was prepared by the reduction of Poly-Ru-1 in the presence of NaBH4 afforded
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1-phenylethy alcohol in the yield of 21% (Table 2, entry 4). This result indicated that
the high catalytic activity of Poly-Ru-1 was attributed to the ruthenium complex
mainly. The most active catalyst, Poly-Ru-1, was selected to investigated the effects
of reaction parameters such as amount of catalyst, time, temperature and type of bases.
When the amount of the catalyst increased from 0.25 to 0.50, 1.25 and 2.5 mol %, the
yields of 1-phenylethyl alcohol were increased from 78 to 91, 96 and 99%,
respectively (Table 2, entry 1, 5-7). With extension of reaction time from 0.5 to 6 h,
the yield of 1-phenylethyl alcohol increased from 42 to 99 % (Table 2, entry 7-11).
o
While the reaction temperature raised from 40 to 82 C, the yields of 1-phenylethy
alcohol increased dramatically from 11 to 99% (Table 2, entry 7, 12, 13), indicating
the high sensitivity of the reaction to temperature. Generally, bases play a vital role in
transfer hydrogenation. Without any bases, the reaction hardly proceeded (Table 2,
entry 14). However, in the presence of KOtBu, the reaction proceeded very well.
With the increase of amount of KOtBu from 6 to 10 mol %, the yield of 1-phenylethy
alcohol increased gradually from 83 to 99% (Table 2, entry 7, 15, 16). KOH also
gave the high yield of 95% (Table 2, entry 17). However, Na2CO3 with lower
alkalinity afforded the product in relative low yield of 9% (Table 2, entry 18). The
strong bases facilitated the formation of ruthenium alkoxide by capturing a proton
from 2-propanol during transfer hydrogenation, and subsequently alkoxide underwent
β-elimination to form ruthenium hydride, an active species in hydrogenation [24-26].
Compared with other reported ruthenium complex catalysts, Poly-Ru-1 had higher
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activity than other heterogeneous catalysts and comparable activity with
homogeneous catalysts (Table S2).
During the transfer hydrogenation reaction, Poly-Ru-1, Poly-Ru-2 and Ru
complex monomer catalysts turned to black gradually. In order to clarify this
phenomenon and further illustrate the reason of the obvious difference of catalytic
activity of these catalysts, the TEM was applied to characterize the Ru species
distribution of Poly-Ru-1, Poly-Ru-2 and Ru complex monomer after reaction. From
the TEM images, there were many ruthenium nanoparticles (Ru NPs) with diameter
of 0.8, 1.5 and 2.5 nm in the recycled catalysts such as Poly-Ru-1 (recycled),
Poly-Ru-2 (recycled) and Ru complex monomer (recycled) (Fig. S3c, d, e). The
relatively smaller particles in Poly-Ru-1 (recycled) may be ascribed to the high
density of DVB (around 80% by mole), which prevented the aggregation of
ruthenium species to form large ruthenium nanoparticles in the polymer matrices. As
the Poly-Ru-2 was prepared by self-polymerization of Ru complex monomer (without
any cross-linker), the ruthenium species were close to each other and aggregated into
Ru NPs easily during the reaction. Furthermore, during the process of reaction
catalyzed by Ru complex monomer, the color of reaction mixture changed into black
in few minutes (Fig. S4), suggesting the reduction of ruthenium complex could
produce Ru NPs in a short time (Fig. S3f) [27]. Correspondingly, homogeneous Ru
complex monomer was easiest to aggregate into larger particles owing to its nature of
untrammeled mobility.
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2.3.2 Transfer hydrogenation of various ketones catalyzed by Poly-Ru-1
To investigate the generalities of heterogeneous Poly-Ru-1 catalyst, various
ketones were applied for transfer hydrogenation under the optimized reaction
conditions (Table 3). The aromatic ketones with electron withdrawing or electron
donating groups had excellent yields of 94-99% (Table 3, entry 1-4). The aliphatic
ketones such as cyclopentanone and 2-pentanone afforded the corresponding alcohols
in the yields of 89% and 92%, respectively (Table 3, entry 5, 6). The benzophenone
achieved a slightly less activity with yield of 83%, presumably ascribed to the steric
hindrance of bulky substrates [28].
3. Conclusion
In conclusion, polymer supported N-heterocyclic carbene ruthenium complex,
Poly-Ru-1, a kind of heterogeneous catalyst, was successfully developed via free
radical polymerization of Ru complex monomer and divinylbenzene (DVB) as the
cross-linker. Poly-Ru-1 had a high specific surface area and good thermal stability.
Poly-Ru-1 had much higher catalytic activity (99% yield) than that of homogenous
catalyst Ru complex monomer (19% yield) in transfer hydrogenation of acetophenone
with 2-propanol. The higher activity of Poly-Ru-1 was attributed to well dispersed
ruthenium species in the polymer which prevented their aggregation. The
heterogeneous catalyst Poly-Ru-1 also showed good generalities for various ketones.
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Table 3. Poly-Ru-1 catalyzed the transfer hydrogenation of various ketones a
Entry
1
Substrate
Product
Yield b (%)
99
2
3
4
5
96
94
95
89
6
7
92
83
aReaction conditions: Ketones (0.24 g, 2 mmol), Poly-Ru-1 (2.5 mol %), 2-propanol
(6 mL), potassium tert-butoxide (10 mol %), 6 h, 82 oC. b Determined by GC.
4. Experimental
Synthesis of Ru complex monomer
The Ru complex monomer was synthesized by transmetallization method with
silver carbene complex. The silver carbene complex (2.23 g, 5 mmol) was dissolved
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in 30 mL anhydrous dichloromethane in a 50 mL round bottom flask equipped with a
magnetic stirrer under the protection of N2. Then the [Ru(p-cymene)Cl2]2 (1.53 g，2.5
mmol) was added into the flask and the reaction mixture was stirred at room
temperature for 12 h in the dark. After reaction, the solvent was removed under
reduced pressure distillation. The orange product was recrystallization in
dichloromethane/pentane (1:5). The desired product was obtained in a yield of 88%
1
(1.40 g). H NMR (400 MHz, CDCl3) δ 7.41 (d, J = 8 Hz, 4H), 7.28 (m, 3H), 6.82 (s,
2H), 6.71 (dd, J = 20, 12 Hz, 2H), 5.86 (s, 2H), 5.77 (d, J = 20 Hz, 2H), 5.62 (s, 2H),
5.37 (d, J = 8 Hz, 2H), 5.27 (d, J = 8 Hz, 2H), 5.06 (d, J = 8 Hz, 2H), 2.85 - 2.92 (m,
1H), 2.04 (s, 3H), 1.24 (d, J = 8 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 175.60,
137.34, 136.85, 136.20, 128.13, 126.66, 122.88, 114.40, 108.01, 97.92, 85.13, 83.31,
54.84, 30.79, 22.61, 18.77. LCMS (ESI, positive ions) calcd for C31H33N2RuCl
[M+H+] 571.1454, found 571.4749.
Synthesis of polymer supported ruthenium complex (Poly-Ru-1)
The Poly-Ru-1 was prepared by radical polymerization of Ru complex monomer
and DVB using AIBN as initiator. Ru complex monomer (1.25 g, 2 mmol), DVB
(1.04 g, 8 mmol) and AIBN (0.12 g, 5 wt%) were stirred at 10 mL refluxing
chloroform for 24 h. The solvent was removed by decantation and the residue was
o
washed with chloroform. The product was dried under vacuum at 50 C for 6 h.
Orange solid was obtained in the yield of 96 % (2.20 g).
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Synthesis of polymer supported ruthenium nanoparticles (Poly-Ru(0))
The polymer supported ruthenium nanoparticles (Poly-Ru(0)) was prepared by
reducing the polymer supported ruthenium complex (Poly-Ru-1) with NaBH4 aqueous
solution. Poly-Ru-1 (0.1 g) and NaBH4 (aq) (10 mL, 1g/L) were stirred at refluxing
for 1 h. The reaction mixture was filtered and the residue was washed with
o
deionization water. The product was dried under vacuum at 50 C for 6 h. Black solid
was obtained in the yield of 99 %.
General experimental procedures for the transfer hydrogenation of ketones
Catalyst Poly-Ru-1 (0.5 mol % based on Ru), base (10 mol %), substrate (2
mmol) and degassed 2-propanol (6 mL) were added to a 50 mL round-bottom flask
with a magnetic stirrer. The reaction mixture was stirred under N2 atmosphere at 82
oC for 6 h. After the reaction mixture was cooled to room temperature, the solid
catalyst was removed by centrifugation and washed with acetone (5×3 mL). The
combined liquid phase was analyzed by GC using mesitylene as an internal standard.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China
(21573072, 21773068, 21811530273), the National Key Research and Development
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Program of China (2017YFA0403102) and Shanghai Leading Academic Discipline
Project (B409).
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Manuscript title: Polymer supported N-heterocyclic carbene ruthenium complex
catalyzed transfer hydrogenation of ketones
Author 1: Rong Wang
 Conceived and designed the analysis
Determined the aim ofresearch and made a plan about this research work.
 Collected the data
Completed all the experiments about the research including preparation of catalyst and application.
 Contributed data or analysis tools
Analyzed the data from experiments using professional chemical software.
 Performed the analysis.
According to the data and results obtained,analyzed and made reasonable explanation.
 Wrote the paper
Wrote the paper and took full participation in revise of manuscript.
 Other contribution
Made the other supplementaryexperiments to explanation the results ofthe experiment.In
addition,made a graphical abstract.
Author 2: Li Qin
☐ Conceived and designed the analysis
 Collected the data
Did the experiments ofBET and SEM of catalysts.
 Contributed data or analysis tools
Helped to analyze the data from BET and SEM of catalysts.
 Performed the analysis
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Discussed and analyzed method for some specific problem.
☐Wrote the paper
 Other contribution
Participated in the process ofrevise.
Author 3: Xin Wang
☐ Conceived and designed the analysis
 Collected the data
Did experiments ofthe XPS of catalysts.
 Contributed data or analysis tools
Helped to analyze the data from XPS of catalysts.
 Performed the analysis
Discussed and analyzed method for some specific problem.
☐ Wrote the paper
 Other contribution
Participated in the process ofrevise.
Author 4: Bihua Chen
☐ Conceived and designed the analysis
☐ Collected the data
 Contributed data or analysis tools
Helped to analyze the data about the catalysis.
 Performed the analysis
Discussed and analyzed method for some specific problem.
☐ Wrote the paper
 Other contribution
Participation to the process ofrevise.
Author 5: Yun Zhao
☐ Conceived and designed the analysis
 Collected the data
Did experiments ofthe GC-MS of catalysts.
 Contributed data or analysis tools
Helped to analyze the data from GC-MS of catalysts.
☐ Performed the analysis
☐ Wrote the paper
☐ Other contribution
Author 6: Guohua Gao
 Conceived and designed the analysis
Developed and designed ofthe research and determined the topic of the research.
☐ Collected the data
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☐ Contributed data or analysis tools
 Performed the analysis
Provided guidance for key problems.
 Wrote the paper
Provided guidance during the process ofrevise aboutthe manuscript.
☐ Other contribution
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Declaration of interests
The authors declare that they have no known competing financial
interests or personal relationships thatcould have appearedto influence
the work reported in this paper.
☐The authors declare the followingfinancial interests/personal relationships which may be
considered as potential competinginterests:
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Highlights:
 Polymer supportedN-heterocycliccarbene ruthenium complex
was prepared.
 The Poly-Ru-1 efficiently catalyzed the transfer
hydrogenation of ketones.
 The Poly-Ru-1 showed superior catalytic activity than the
Ru complex monomer.
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