Metalated Mesoporous Poly(triphenylphosphine) with Azo Functionality: Efficient Catalysts for CO2 Conversion

Article abstract of DOI:10.1021/acscatal.5b02583

Mesoporous poly(triphenylphosphine) with azo functionality (poly(PPh₃)-azo) is reported, which was synthesized via oxidative polymerization of P(m-NH₂Ph)₃ at ambient conditions. This kind of polymer could strongly coordinate with metal ions (e.g., Ru³⁺) and could reduce Ag⁺ in situ to metallic form. The resultant metalated poly(PPh₃)-azo (e.g., poly(PPh₃)-azo-Ag or -Ru) were discovered to be highly efficient catalysts for CO₂ transformation. Poly(PPh₃)-azo-Ag showed more than 400 times higher site-time-yield (STY) for the carboxylative cyclization of propargylic alcohols with CO₂ at room temperature compared with the best heterogeneous catalyst reported. Poly(PPh₃)-azo-Ru also exhibited good activity for the methylation of amines with CO₂. It was demonstrated that the high performances of the catalysts originated from the cooperative effects between the polymer and the metal species. In addition, both catalysts showed good stability and easy recyclability, thus demonstrating promising potential for practical utilization for the conversion of CO₂ into value-added chemicals.

Full text of DOI:10.1021/acscatal.5b02583

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Article
Metalated mesoporous poly(triphenylphosphine) with
azo-functionality: Efficient catalysts for CO2 conversion
Zhen-zhen Yang, Bo Yu, Hong-ye Zhang, Yan-fei Zhao, Yu Chen,
Zhishuang Ma, Guipeng Ji, Xiang Gao, Buxing Han, and Zhi-min Liu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02583 • Publication Date (Web): 07 Jan 2016
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Metalated mesoporous poly(triphenylphosphine) with azo-
functionality: Efficient catalysts for CO2 conversion
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Zhenzhen Yang, Bo Yu, Hongye Zhang, Yanfei Zhao, Yu Chen, Zhishuang Ma, Guipeng Ji, Xiang
Gao, Buxing Han and Zhimin Liu*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Thermodynamics, In-
stitute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
ABSTRACT: Mesoporous poly(triphenylphosphine) with azo functionality (poly(PPh3)-azo) was reported, which was
synthesized via oxidative polymerization of P(m-NH2Ph)3 at ambient conditions. This kind of polymer could strongly co-
ordinate with metal ions (e.g., Ru3+), and could in-situ reduce Ag+ to metallic form. The resultant metalated poly(PPh3)-
azo (e.g., poly(PPh3)-azo-Ag or -Ru) were discovered to be highly efficient catalysts for CO2 transformation.
Poly(PPh3)-azo-Ag showed more than 400 times higher site-time-yield (STY) for the carboxylative cyclization of propar-
gylic alcohols with CO2 at room temperature compared with the best heterogeneous catalyst reported. Poly(PPh3)-azo-
Ru also exhibited good activity for the methylation of amines with CO2. It was demonstrated that the high performances
of the catalysts were originated from the cooperative effects between the polymer and the metal species. In addition, both
catalysts showed good stability and easy recyclability, thus demonstrating promising potential for practical utilization of
the cost-effective process for the conversion of CO2 into value-added chemicals.
KEYWORDS: Porous organic polymer • CO2 conversion • α-alkylidene cyclic carbonate • methylation of amine • hetero-
geneous catalysis
As an emerging material platform, porous organic pol-
ymers (POPs) have attracted considerable scientific inter-
INTRODUCTION
Carbon dioxide chemistry (e.g. capture and conversion)
has attracted much attention from the scientific commu-
nity due to global warming associated with positive car-
bon accumulation.1 In comparison with toxic phosgene
and CO, CO2 is a cheap, green and renewable C1 building
block due to its abundance, availability and nontoxicity,
thus the CO2-involved synthesis of chemicals has been
regarded as green approaches.2 The major challenge in
CO2 transformation originates from the inherent thermo-
dynamic stability and kinetic inertness of CO2, which calls
for a remarkable driving force to ensure efficient trans-
formation.3 In this respect, the key issue to convert CO2
into useful chemicals inevitably relies on its activation.4
With the rapid development of organometallic chemistry
and catalysis, various types of homogeneous and hetero-
geneous catalysts for CO2 transformation have been re-
ported in the past decades.4 In general, phosphine or ni-
trogen-containing ligands are involved in most of the
homogeneous organometallic catalysts, in order to pro-
mote the reactivity and also control the selectivity of CO2
chemical fixation.4 Nevertheless, the heterogeneous cata-
lysts are more desirable for industrial applications owing
to the convenience of recovering and recycling of the cat-
alysts. However, low catalytic activity and poor selectivity
together with leaching of active species were generally
observed in the case of heterogeneous catalysts. Therefore,
design of highly efficient heterogeneous catalysts for CO2
conversion is still a great challenge.
est due to their distinctive properties such as large surface
areas, low skeleton density, good physicochemical and
thermal stability. Most importantly, POPs could be de-
signed with various functional components, showing wide
applications in many areas.5-8 For example, the POPs with
nitrogen-rich functionalities, such as azo,9-11 triazine,12
tetrazole,13 imidazole14 and amine15 species showed "CO2-
philic" properties and superior capacity to CO2 uptake,
which is generally believed to arise from enhanced CO2-
framework interactions. Recently, the POPs that can co-
ordinate with metal species have been reported, which
widen the applications of POPs in catalysis.16-25 For in-
stance, Tröger's base-derived POPs could coordinate with
Ru species, showing high efficiency for the hydrogenation
of CO2 to produce HCOOH;17 Ru coordinated azo-
functionalized POPs displayed high performances for
catalyzing the methylation of amines using CO2.18 Howev-
er, in these systems, additional high amount of organic
ligands (e.g., PPh3) were still required to achieve a high
efficiency of the catalysts. Hence, if POPs are designed to
possess both CO2-philic moieties and organic ligands, the
resultant metalated POPs may have very high ligand con-
centration as well as the ability for CO2 activation, and
thus may exhibit high performance as heterogeneous cat-
alysts for CO2 transformation.
Herein, we designed porous poly(triphenylphosphine)
with azo functionality (poly(PPh3)-azo), which was syn-
thesized through oxidative polymerization of phosphine-
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containing aromatic amines, P(m-NH2Ph)3, in the pres-
ence of t-BuOCl/NaI at 25 C (Scheme 1). Notably, the
polarization magic-angle spinning (CP/MAS) 13C NMR
o
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spectra (Figure S3, SI) of poly(PPh3)-azo located in the
range of 119.7-151.4 ppm belonged to the aromatic carbons
in the backbone. The thermogravimetric analysis (TGA)
results (Figure S4, SI) indicated that poly(PPh3)-azo was
reaction proceeded rapidly and afforded almost quantita-
tive product yield. The resultant poly(PPh3)-azo could
coordinate with transition metal ions (e.g., Ru3+), and
interestingly, it could in-situ reduce Ag+ to metallic form.
The resultant metalated poly(PPh3)-azo catalysts dis-
played high efficiency for catalysing CO2 transformation,
originated from the cooperative effects between the pol-
ymer and the metal species. The Ag immobilized
poly(PPh3)-azo (poly(PPh3)-azo-Ag) could catalyse the
carboxylative cyclization of propargyl alcohols with CO2
at room temperature, affording more than 400 times
higher site-time-yield (STY) compared with the best het-
erogeneous catalytic system reported. The Ru coordinat-
ed poly(PPh3)-azo (poly(PPh3)-azo-Ru) also exhibited
extraordinary activity for the methylation of amines with
CO2 under low pressure. Moreover, both poly(PPh3)-azo-
Ag and poly(PPh3)-azo-Ru showed good stability and
easy recyclability, thus demonstrating great potential for
practical utilization in catalysis.
o
stable in air up to 400 C, which met the demands for
potential applications in heterogeneous catalysis. The N2
sorption measurements at 77 K of poly(PPh3)-azo
showed a Brunauer-Emmett-Teller (BET) surface area and
the total pore volume of 118 m2 g-1 and 0.62 cm3 g-1, respec-
tively (Figure 1A and Figure S5A, SI). The pore size distri-
bution suggested that the porosity was dominated by
mesopores (Figure 1B). Both the structure of the mono-
mer and the formed azo functionality are not sufficiently
rigid to prevent intermolecular packing, which resulted in
a relatively low BET surface area. Scanning electron mi-
croscopy (SEM, Figure S6, SI) and transmission electron
microscopy (TEM, Figure 1C and S7) images of
poly(PPh3)-azo further confirmed the existence of meso-
pores in the polymer particles and very small amount of
macropores due to the aggregation of the particles.
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NH2
H2N
P
P(m-NH2Ph)3
NH2
t-BuOCl/NaI 25 oC, 1 h
N
N
N
P
N
N
N
N
N
N
P
N
P
N
N
N
N
poyl(PPh3)-azo
Figure 1. For poly(PPh3)-Azo: A) Adsorption (filled) and
desorption (empty) isotherms of N2 at 77 k; B) Pore size dis-
tribution curve obtained from the adsorption branches using
non-local density functional theory (NLDFT) method; C)
TEM image. D) TEM image for poly(PPh3)-azo-Ag.
Scheme 1 Synthetic process of poly(PPh3)-azo through
oxidative polymerization of the monomer P(m-NH2Ph)3.
RESULTS AND DISCUSSION
Both the azo- and phosphine-type ligands are versatile
in coordination chemistry.18,26,27 In this work, the resultant
poly(PPh3)-azo was a polymer composed of PPh3 con-
nected with large amount of azo bonds, which may have
unique features. Ag or Ru metalated poly(PPh3)-azo
were obtained by treating the polymer with AgBF4 or
RuCl3·3H2O in tetrahydrofuran (THF) or ethanol (EtOH),
respectively, under refluxing conditions (for synthetic
process, see SI). The contents of Ag and Ru species im-
mobilized onto poly(PPh3)-azo were 0.17 wt% and 3.72
wt%, respectively, determined by inductively coupled
plasma-optical emission spectroscopy (ICP-OES) analysis.
The energy dispersive spectrometer (EDS) profile ob-
tained during the TEM observation also indicated the
The formation of poly(PPh3)-azo was revealed by Fou-
rier transform infrared (FTIR) spectroscopy (Figure S1, SI).
Compared with the monomer P(m-NH2Ph)3, the presence
of -N=N- functionality was confirmed by the characteris-
tic bands at 1175 and 1410 cm-1 due to the symmetric and
asymmetric vibrations of the azo group. The broad bands
located at 3395 cm-1 corresponded to the N-H stretching
mode, suggesting the presence of unreacted terminal
amino groups.9-11,18 The solid-state ultraviolet-visible anal-
ysis for poly(PPh3)-azo showed an intense new absorp-
tion band at 440 nm, which revealed that cis configura-
tion of azo functionality predominantly existed (Figure S2,
SI).11 The chemical shifts in the solid-state cross-
2
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were examined in carboxylative cyclization of propargyl
presence of metal species (Figure S9, SI). Both of the
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metallated polymeric materials exhibited good thermal
stability confirmed by TGA analysis (Figure S4, SI). Com-
pared with those of poly(PPh3)-azo (118 m2 g-1 and 0.62
cm3 g-1), both the BET surface areas and total pore vol-
umes of the metallated polymers decreased: 80 m2 g-1 and
0.53 cm3 g-1 for poly(PPh3)-azo-Ag, and 78 m2 g-1 and 0.40
cm3 g-1 for poly(PPh3)-azo-Ru (Figures S5 and S8, SI).
This was probably due to the weight increase and the
slight block of the cavities by metal species.
alcohols and methylation of amines with CO2, respective-
ly. The carboxylative cyclization of propargyl alcohols
with CO2 is a green approach to synthesize α-alkylidene
cyclic carbonates that have potential bioactivity with a
broad range of applications as intermediates in organic
synthesis.32-34 Heterogeneous catalysts including (dime-
thylamino)-methyl-polystyrene-supported CuI (DMAM-
PS-CuI),35 polystyrene-supported NHC-Ag complexes (PS-
NHC-Ag)36 have been reported for this reaction. However,
high CO2 pressure (up to 14 MPa), high catalyst loading of
transition metals (up to 10 mol%) together with very low
STY (< 1 h-1) were generally the cases. Poly(PPh3)-azo-Ag
prepared in this work was found to show very high per-
formance for catalysing the cyclization of propargyl alco-
hols with CO2 at room temperature, as illustrated in Table
1 and Scheme 2.
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X-ray photoelectron spectroscopy (XPS) measurements
were performed to investigate the coordination site and
oxidation state of N, P and metal species. Vast majority of
the incorporated N could be regarded as stable structural
azo nitrogen with N1s binding energy (BE) at 399.97 eV
(Figure S10, SI).28 The other two peaks with the BEs of
397.87eV and 403.22 eV for N1s were associated to unre-
acted aryl amino group and quaternary-type nitrogen
formed during the oxidative process as intermediate, re-
spectively.29 The presence of P in the backbone of
poly(PPh3)-azo was confirmed by the peaks at 132.32 eV
(P2p3/2) and 133.1 eV (P2p1/2). The XPS spectrum of
poly(PPh3)-azo-Ag demonstrated two peaks of Ag3d5/2
and Ag3d3/2 with BEs at 368.12 and 374.12 eV, respectively
(Figure S11, SI), ascribing to metallic Ag species, which
suggested that the polymer could in-situ reduce Ag+ to
metallic Ag0, probably by the residual aryl amino
groups.30,31 The TEM image also supported the formation
of Ag0 nanoparticles, which were evenly distributed
throughout the polymer support and the particle sizes
were centred around 2.7 nm (Figure 1D). The BE of N1s in
poly(PPh3)-azo-Ag exhibited a slightly higher value of
400.07 eV than that in poly(PPh3)-azo (i.e., 399.97 eV),
while the BE of P2p showed no difference. This implies
that Ag nanoparticles were prone to nucleate and grow at
the azo sites in the as-prepared poly(PPh3)-azo-Ag. Due
to the uniform presence of azo sites throughout the pol-
ymer, the Ag nanoparticles were uniformly distributed.
Notably, only 0.17 wt% Ag species were immobilized onto
the polymer support in poly(PPh3)-azo-Ag, meaning Ag
particles were embedded with PPh3 ligand and azo func-
tionality in high concentration, which was important for
catalytic CO2 conversion as shown below.
As a model reaction the cyclization of 2-emthylbut-3-
yn-2-ol (1a) with CO2 was investigated over poly(PPh3)-
azo-Ag and different catalytic systems. The control exper-
iments indicated that nearly no product was detected
without catalyst (Table 1, entry 1), and poly(PPh3)-azo
showed no catalytic activity either (entry 2). Excitingly,
quantitative yield of α-alkylidene cyclic carbonate (2a)
was obtained by employing only 0.064 mol% poly(PPh3)-
azo-Ag as the catalyst, giving a STY of 87 h-1 for the pro-
duction of 2a (entry 3). The evolution of the catalytic per-
formance of poly(PPh3)-azo-Ag along with reaction time
indicated that the STY values increased with shortening
the time, and the highest STY value of 469 h-1 was ob-
tained at the reaction time of 1 h, together with 2a yield of
30%. A comparable STY value of 438 h-1 was observed by
further reducing the reaction time to 0.5 h (entries 3-8). It
was noteworthy that the STY value of 182 h-1 achieved by
poly(PPh3)-azo-Ag was much higher than that obtained
over the homogeneous AgBF4 catalyst with 10 mol% load-
ing (1.5 h-1) within the same reaction time of 6 h (entries 5
vs 9). To achieve a comparable yield of 2a (30%) by
poly(PPh3)-azo-Ag (0.064 mol%) within 1 h, at least 2
mol% of AgBF4 was required, yet with an inferior STY
value (13 h-1) (entries 7 vs. 10). In addition, the advantage
of poly(PPh3)-azo-Ag was prominent compared with the
reported heterogeneous catalytic systems bearing Ag (1 h-1)
or Cu species (0.52 h-1) even under harsh reaction condi-
tions (Table 1, entries 11, 12). To explore the reasons of the
high performance of poly(PPh3)-azo-Ag, control experi-
ments were performed using AgBF4 as the catalyst with
azobenzene or/and PPh3 as additives. It was demonstrat-
ed that azobenzene and PPh3 in the AgBF4- catalyzed re-
action process could improve the yields of 2a (entries 10
vs. 13-16). Particularly, the coexistence of azobenzene and
PPh3 increased the STY values from 28 to 37 h-1 by increas-
ing the molar ratio of additives to AgBF4 (entry 15 vs. 16).
These results suggested that the presence of azobenzene
or/and PPh3
Interestingly, Ru species were coordinated with both
azo and phosphine functionalities in poly(PPh3)-azo-Ru,
which was confirmed by the higher BEs of N1s (400.02 eV)
and P2p3/2 (132.42 eV) compared with those in the support
poly(PPh3)-azo (Figure S12, SI). The four peaks with the
BEs of 281.57 eV (Ru3d5/2), 285.62 eV (Ru3d3/2, overlapping
with the peak for C1s), 463.07 eV (Ru3p3/2) and 485.17 eV
(Ru3p1/2) were consistent with those of the Ru species in
the +3 state, however, shifting to lower values compared
to those of RuCl3, suggesting the coordination of Ru(III)
with the polymer.18 In addition, the TEM image supported
a very homogeneous ruthenium distribution without de-
tectable ruthenium clusters or nanoparticles in
poly(PPh3)-azo-Ru (Figure S13, SI).
Table 1. Carboxylative cyclization of propargyl alcohol
a
(1a) with CO2
Ag- and Ru-based catalysts have been applied in the
CO2 transformation. In this work, the catalytic perfor-
mances of poly(PPh3)-azo-Ag and poly(PPh3)-azo-Ru
3
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the metallic form (Figure S11, SI). For leaching text, excess
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8
of CO2 was vented and the heterogeneous catalyst
poly(PPh3)-azo-Ag was separated through centrifugation
after the reaction being performed for 1 h, and the filtrate
was detected by ICP-OES. It was demonstrated that Ag
species were undetectable (< 10 ppb). All these indicated
the excellent stability and activity of poly(PPh3)-azo-Ag
as a heterogeneous catalyst for the conversion of CO2 at
room temperature.
Entry Catalyst
Time
/h
18
18
18
12
6
Yield of STY/
2a /%b
h-1c
1
No
<1
-
9
2
poly(PPh3)-azo
poly(PPh3)-azo-Ag
poly(PPh3)-azo-Ag
poly(PPh3)-azo-Ag
poly(PPh3)-azo-Ag
poly(PPh3)-azo-Ag
poly(PPh3)-azo-Ag
AgBF4
<1
-
Numerous terminal propargylic alcohols with both al-
kyl and aryl substitutes at the propargylic position could
react with CO2 efficiently, giving >90% yields of the corre-
sponding α-alkylidene cyclic carbonates (Scheme 2). Es-
pecially, products with bulky isopropyl group (2d) or
small rings (2f and 2g) were obtained in high yields alt-
hough relatively longer reaction time was required due to
steric hindrance effects. The extraordinary performance
of poly(PPh3)-azo-Ag was probably ascribed to the fol-
lowing aspects: (1) high ligand concentration (azo and
phosphine ligands) around the Ag particles; (2) uniform
distribution and stable immobilization of Ag nanoparti-
cles; (3) porous structure of the polymer; (4) CO2-philic
property of the azo group. A possible reaction mechanism
for the reaction of CO2 with propargyl alcohol catalyzed
by poly(PPh3)-azo-Ag was proposed as shown in Scheme
S1 (SI).
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3
100
88
70
56
30
14
87
115
182
292
469
438
1.5
13
4
5
6
3
7
1
8
0.5
6
9d
10e
11f 36
12g 35
13h
14h
15i
88
26
96
99
31
AgBF4
1
PS-NHC-Ag
24
24
1
1
DMAM-PS-CuI
AgBF4/azobenzene
AgBF4/PPh3
0.52
16
1
44
56
73
22
28
37
AgBF4/azobenzene/PPh3
AgBF4/azobenzene/PPh3
1
16i
1
a
Reaction conditions: 1a 0.5 mmol, poly(PPh3)-azo-Ag
0.02 g, 0.064 mol% Ag based on 1a, DBU 0.5 mmol, THF 2
o
b
1
mL, CO2 pressure 1 MPa, 25 C, 18 h; Determined by H
NMR (CDCl3, 400 MHz) using 1,4-bis(chloromethyl)-
benzene as an internal standard (Figure S14, SI); STY =
c
[mol product obtained]/[(total mol metal)*(reaction
d
e
f
time)]; AgBF4 10 mol%; AgBF4 2 mol%; Catalyst load-
o
ing 2 mol%, CO2 pressure 5 MPa, 40 C, 24 h; Data from
36
g
Ref.
;
Catalyst loading 8 mol%, CO2 pressure 14 MPa,
o
35
h
40 C, 24 h; Data from Ref.
;
AgBF4 2 mol%, azoben-
i
zene 0.15 mmol (entry 13), PPh3 0.1 mmol (entry 14);
AgBF4 2 mol%, azobenzene 0.075 mmol (entry 15) or 0.15
mmol (entry 16), PPh3 0.05 mmol (entry 15) or 0.1 mmol
(entry 16).
promoted the catalytic activity of Ag catalyst remarkably.
In the as-prepared poly(PPh3)-azo-Ag, PPh3 and azo
functionalities dominated the composition of the polymer,
generating very high molar ratios of ligand to Ag nano-
particles, approximately 200: 1 for PPh3 to Ag and 300: 1
for azo to Ag. In addition, strong interaction existed be-
tween the Ag particles and the polymer support. All these
generated cooperative effect for catalysing the transfor-
mation of CO2, responsible for the high efficiency of the
catalyst.37
Scheme 2 Substrate scope for the reaction of propargyl
alcohols with CO2 catalyzed by poly(PPh3)-azo-Ag. Reac-
tion conditions: propargyl alcohols 0.5 mmol,
poly(PPh3)-azo-Ag 0.02 g, 0.064 mol% Ag based on 1,
DBU 0.5 mmol, THF 2 mL, CO2 pressure 1 MPa, 25 oC.
In the repeating reactions, the catalytic activity of
poly(PPh3)-azo-Ag was well retained after recycling the
catalyst for five times (Figure S15, SI). The TEM images
indicated that the metallic Ag particles were still highly
dispersed without the changes in particle size after recy-
cling (Figure S16B, SI). The XPS analysis on the recycled
poly(PPh3)-azo-Ag showed that the Ag species were in
The cooperative effect between the resultant polymer
poly(PPh3)-azo and the coordinated metal species was
Table 2. Poly(PPh3)-azo-Ru catalyzed methylation of
a
amines using CO2
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Yield/%b
99
cooperative effects of poly(PPh3)-azo with the coordi-
Entry Substrate
Product
1
2
3
nated Ru species resulted in the high efficiency of
poly(PPh3)-azo-Ru. The proposed reaction mechanism
was shown in Scheme S2.
1
4
5
6
7
8
9
2
3
4
5
6
7
8
26c
64d
92
90
95
Although reductive hydrosilane existed in the reaction
system, the coordinated Ru species was stable enough to
maintain in the +3 state and homogenous dispersion
without detectable Ru clusters after five cycles, confirmed
by TEM observation (Figure S16A, SI) and XPS analysis on
the recycled Poly(PPh3)-azo-Ru (Figure S12, SI).
Poly(PPh3)-azo-Ru demonstrated good chemical stabil-
ity and easy recyclability as well, as shown in Figure S17,
consistent to poly(PPh3)-azo-Ag. No Ru species in the
filtrate after separating the solid catalyst from the reac-
tion mixture were detectable by ICP-OES, further indicat-
ing that poly(PPh3)-azo-Ru was stable. Moreover,
poly(PPh3)-azo-Ru could effectively catalyse the methyl-
ation of various N-methylanilines with both electron-
donating (entries 4 and 5)/electron-withdrawing groups
(entries 6 and 7) and dialkylamine (entry 8) with CO2,
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affording
the
corresponding
substituted
N,N-
dimethylanilines in >90% yields.
CONCLUSIONS
In summary, mesoporous poly(PPh3) with azo func-
tionality was obtained through simple oxidative polymer-
ization of P(m-NH2Ph)3 under ambient conditions, which
was a highly efficient supports for transition metal species.
The resultant metalated poly(PPh3)-azo demonstrated
extraordinary performances for the conversion of CO2
into value-added chemicals (e.g. α-alkylidene cyclic car-
bonates and methylamines), together with high stability
and easy recyclability. The cooperative effects of
poly(PPh3)-azo and the metal species were responsible for
the high performances of the catalysts for CO2 transfor-
mation. This work provides multifunctional catalysts for
CO2 conversion, which may have great potential in appli-
cations.
a
Reaction conditions: substrate 0.5 mmol, catalyst load-
ing (poly(PPh3)-azo-Ru) 1 mol% Ru based on the sub-
strate, organosilane PhSiH3 4 mmol, CO2 pressure 0.5
MPa, THF 2 mL, 120 C, 24 h; Isolated yield; Catalyst
loading (Azo-MOP-3-Ru) 1 mol% Ru based on the sub-
strate, additive PPh3 0.05 mmol; RuCl3·3H2O 4 mol%,
PPh3 0.1 mmol.
o
b
c
d
also reflected by the high performance of poly(PPh3)-
azo-Ru for catalyzing the methylation of amines with
CO2. The methylation of amines with CO2 in the presence
of hydrosilanes is a promising method to produce me-
thylamines that are basic reagents with many usages in
nitrogen chemistry.38,39 Homogeneous N-heterocyclic
carbenes (NHCs) zinc complex,38 ruthenium complex,39
NHCs40 catalysts have been developed to afford N-
methylated compounds using CO2 as a C1 building block.
However, these catalytic systems all needed to be manip-
ulated under inert atmosphere. The heterogeneous cata-
lysts for this kind of reactions under mild conditions are
rarely reported.18,41 Although Ru-coordinated Azo-MOPs
(e.g., Azo-MOP-3-Ru) reported in our previous work
displayed activity for this kind of reactions, tremendous
PPh3 was required.18 Excitingly, poly(PPh3)-azo-Ru pre-
pared in this work was found to be very effective for cata-
lysing the methylation of amines with CO2 without any
additive at low CO2 pressure, as listed in Table 2. In con-
trast, 1 mol% of Azo-MOP-3-Ru together with 10 equiv. of
PPh3 (that is, the Ru:PPh3 ratio was identical to that in
poly(PPh3)-azo-Ru) only afforded a yield of 26% (entry 2,
Table 2), meaning that poly(PPh3)-azo-Ru showed near
4 times higher activity than Azo-MOP-3-Ru under the
similar conditions. Moreover, poly(PPh3)-azo-Ru
showed activity even much better than the homogenous
RuCl3·3H2O catalyst (4 mol%) combined with PPh3 (entry
1 vs. 3). From these findings, it can be deduced that the
ASSOCIATED CONTENT
Supporting Information available: general experi-
mental methods, synthetic procedures, Figure S1-S17,
Scheme S1, S2 and characterization (NMR) of the α-
alkylidene cyclic carbonates and methylamine products.
AUTHOR INFORMATION
Corresponding Author
*E-mail: liuzm@iccas.ac.cn
ACKNOWLEDGMENT
This work was financially supported by the National
Natural Science Foundation of China (Grants Nos.
21533011, 21125314, 21402208).
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