Imidazolium-based organoiridium-functionalized periodic mesoporous organosilica boosts enantioselective reduction of α-cyanoacetophenones, α-nitroacetophenones, and β-ketoesters

Article abstract of DOI:10.1016/j.jcat.2014.09.019

An imidazolium-based, organoiridium-functionalized periodic mesoporous organosilica is developed through complexation of chiral pentafluorophenylsulfonyl-1,2-diphenylethylenediamine and organoiridium-functionalized periodic mesoporous organosilica. Structural analyses and characterizations of catalyst reveal well-defined single-site iridium active species within its organosilicate network. Electron microscopy confirms a highly ordered dimensional-hexagonal mesostructure. This bifunctional heterogeneous catalyst displays excellent catalytic performance in the enantioselective reduction of α-cyano and α-nitroacetophenones. As expected, incorporation of imidazolium-functionality within hydrophobic periodic mesoporous organosilica promotes catalytic activity and enantioselectivity. In addition, this heterogeneous catalyst can be recovered and reused for at least eight times without loss of its catalytic activity. Furthermore, the approach described here can also construct another organoiridium-functionalized periodic mesoporous organosilica through postcoordination of chiral methylsulfonyl-1,2-diphenylethylenediamine, which provides excellent catalytic activity and enantioselectivity in the enantioselective reduction of β-ketoesters. The method presented here offers a potential way for immobilizing various chiral ligands to construct chiral organometal-functionalized periodic mesoporous organosilicas.

Full text of DOI:10.1016/j.jcat.2014.09.019

Journal of Catalysis 320 (2014) 70–76
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Imidazolium-based organoiridium-functionalized periodic mesoporous
organosilica boosts enantioselective reduction
of
a
-cyanoacetophenones,
a-nitroacetophenones, and b-ketoesters
⇑
Boxin Deng, Wei Xiao, Cuibao Li, Feng Zhou, Xuelin Xia, Tanyu Cheng, Guohua Liu
Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An imidazolium-based, organoiridium-functionalized periodic mesoporous organosilica is developed
through complexation of chiral pentafluorophenylsulfonyl-1,2-diphenylethylenediamine and organoiri-
dium-functionalized periodic mesoporous organosilica. Structural analyses and characterizations of cat-
alyst reveal well-defined single-site iridium active species within its organosilicate network. Electron
microscopy confirms a highly ordered dimensional-hexagonal mesostructure. This bifunctional heteroge-
Received 4 May 2014
Revised 22 September 2014
Accepted 28 September 2014
neous catalyst displays excellent catalytic performance in the enantioselective reduction of
a-cyano and
Keywords:
a
-nitroacetophenones. As expected, incorporation of imidazolium-functionality within hydrophobic peri-
Asymmetric catalysis
Heterogeneous catalysis
Immobilization
Mesoporous materials
Silica
odic mesoporous organosilica promotes catalytic activity and enantioselectivity. In addition, this hetero-
geneous catalyst can be recovered and reused for at least eight times without loss of its catalytic activity.
Furthermore, the approach described here can also construct another organoiridium-functionalized peri-
odic mesoporous organosilica through postcoordination of chiral methylsulfonyl-1,2-diphenylethylen-
ediamine, which provides excellent catalytic activity and enantioselectivity in the enantioselective
reduction of b-ketoesters. The method presented here offers a potential way for immobilizing various chi-
ral ligands to construct chiral organometal-functionalized periodic mesoporous organosilicas.
Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction
constructing PMO through assembly often requires a chiral
ligand-derived silane [14–17]. However, the complicated synthetic
process and tedious purification steps still limit their practical
applications; chiral ligand-derived silanes are difficult to purify
by silica gel column chromatography. Thus, utilizing the approach
of direct postcoordination of chiral ligands may be a worthwhile
alternative for constructing chiral organometal-functionalized
PMOs, which has not been exported yet.
In our effort to develop various heterogeneous catalysts based
on mesoporous silica [18–24], we found that several chiral organo-
metal-functionalized PMOs have high hydrophobicity and could
greatly promote asymmetric reactions [18,19]. We also found that
the imidazolium-functionality within an organic–inorganic hybrid
silica could significantly enhance the performance of a biphase cat-
alytic reaction [20–22]. In the present contribution, we assembled
Development of periodic mesoporous organosilicas (PMO) sup-
ports to immobilize various chiral organometallic complexes for
asymmetric catalysis has attracted much interest [1,2]. Similar to
inorganosilica mesoporous materials such as SBA-15 and MCM-
41 [3,4], PMOs also possess large specific surface area and pore
volume, tunable pore dimension, well-defined pore arrangement,
as well as high thermal and mechanical stability. But in contrast
to inorganosilicate mesoporous materials, PMOs have high hydro-
phobicity due to their intrinsic organosilicate inner surface, a sig-
nificant advantage as it greatly promotes organic transformation
[5,6]. Moreover, a PMO with imidazolium functionality in its orga-
nosilicate network can synergistically facilitate an organic reaction
in a biphase catalytic system [7–9]. Although many achiral PMOs,
including some imidazolium-based PMOs, have been reported
[10–13], incorporation of the imidazolium functionality in a chiral
ligand- or organometal-functionalized PMO network for asymmet-
ric catalysis is still rare. Furthermore, general strategy for
a
hydrophobic, imidazolium-based PMO by incorporating an
imidazolium functionality in an ethylene-bridged PMO. Taking
advantage of the postcoordination of (S,S)-pentafluorophenylsulfo-
nyl-1,2-diphenylethylenediamine (PFPSDPEN), we developed a
hydrophobic, imidazolium-based, organoiridium-functionalized
heterogeneous catalyst. This catalyst efficiently facilitated the
⇑
Corresponding author. Fax: +86 21 64321819.
E-mail address: ghliu@shnu.edu.cn (G. Liu).
enantioselective reduction of
a-cyanoacetophenones and
http://dx.doi.org/10.1016/j.jcat.2014.09.019
0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

B. Deng et al. / Journal of Catalysis 320 (2014) 70–76
71
a
-nitroacetophenones in aqueous medium. As expected, the sali-
H
1.23,
N
0.61,
S
1.41. SBET
:
740.0 m2/g, dpore
:
8.02 nm,
ent imidazolium-functionality and the high hydrophobicity of the
PMO synergistically boosted the catalytic performance in aqueous
medium. In addition, the heterogeneous catalyst could be
recovered and reused for at least eight times without loss of its cat-
alytic activity. Furthermore, applying the approach of direct post-
coordination of chiral ligands, we also constructed another chiral
organoiridium-functionalized heterogeneous catalyst through
postcoordination of (S,S)-methylsulfonyl-1,2-diphenylethylenedi-
amine and imidazolium-based PMO, which efficiently catalyzed
the highly enantioselective reduction of b-ketoesters in aqueous
medium.
Vpore: 0.90 cm3/g. 29Si MAS NMR (79.4 MHz): T1 (d = À49.3 ppm),
T2 (d = À58.3 ppm), T3 (d = À64.7 ppm). 13C CP/MAS NMR
(161.9 MHz): 137.4, 124.1 (CH of imidazolium), 56.5 (AOCH2CH3),
47.3 (ANCH2CH2CH2SA), 37.1 (ANCH2CH2CH2SiA), 21.9 (AOCH2-
CH3), 16.4 (ANCH2CH2CH2SA), 8.1–0.5 (ACH2Si) ppm.
2.2.2. Preparation of the heterogeneous catalyst 5
In a typical synthesis, Ag2O (115.5 mg, 0.5 mmol) was added to a
suspension of HO3S-PMO (3) (1.00 g) in 20.0 mL of distilled water at
room temperature, and the resulting mixture was stirred at 90 °C
for 6 h. After cooling to ambient temperature, a solution of 2.0 N
H2SO4 (10 mL) was added and the mixture was stirred overnight.
The solids were collected by centrifugation and washed repeatedly
with excess distilled water. The collected light-gray solids were
suspended in a solution of 10.0 mL of methanol and 10.0 mL dis-
tilled water, and 159.2 mg (0.20 mmol) of [Cp⁄IrCl2]2 was added
to the solution at ambient temperature. The resulting mixture
was stirred for 12 h. Finally, 176.8 mg (0.40 mmol) of (S,S)-penta-
fluorophenylsulfonyl-1,2-diphenylethylenediamine was added
and the mixture was stirred further for 6 h at ambient temperature.
The mixture was filtered through filter paper and then rinsed with
excess water and CH2Cl2. After Soxhlet extraction for 24 h in CH2Cl2
to remove homogeneous and unreacted starting materials, the solid
was dried at ambient temperature under vacuum overnight to
afford catalyst 5 (1.13 g) as a light-yellow powder. ICP analysis
showed that the Ir loading was 44.16 mg (0.23 mmol) per gram of
catalyst and the residual Ag amount was 8.51 mg (0.078 mmol)
per gram of catalyst. IR (KBr) cmÀ1: 3426.7 (s), 2983.0 (w), 2896.1
(w), 1634.6 (m), 1491.6 (w), 1412.8 (w), 1261.7 (m), 1158.8 (s),
1031.8 (s), 912.9 (m), 769.9 (m), 697.8 (m) 441.7 (m). Elemental
2. Experimental
2.1. Characterization
Ir loading amount in the catalyst was analyzed using an induc-
tively coupled plasma optical emission spectrometer (ICP, Varian
VISTA-MPX). Fourier transform infrared (FT-IR) spectra were col-
lected on a Nicolet Magna 550 spectrometer using KBr method.
X-ray powder diffraction (XRD) was carried out on a Rigaku D/
Max-RB diffractometer with Cu K
a radiation. Scanning electron
microscopy (SEM) images were obtained using a JEOL JSM-
6380LV microscope operating at 20 kV. Transmission electron
microscopy (TEM) images were performed on a JEOL JEM2010 elec-
tron microscope at an acceleration voltage of 220 kV. X-ray photo-
electron spectroscopy (XPS) measurements were taken on
Perkin-Elmer PHI 5000C ESCA system. A 200 m diameter spot size
was scanned using a monochromatized Aluminum K X-ray source
(1486.6.6 eV) at 40 W and 15 kV with 58.7 eV pass energies. All the
binding energies were calibrated by using the contaminant carbon
(C1s = 284.6 eV) as a reference. Nitrogen adsorption isotherms were
measured at 77 K with a Quantachrome Nova 4000 analyzer. The
samples were measured after being outgassed at 423 K overnight.
Pore size distributions were calculated by using the BJH model. The
specific surface areas (SBET) of samples were determined from the
linear parts of BET plots (p/p0 = 0.05–1.00). Thermal gravimetric
analysis (TGA) was performed with a Perkin-Elmer Pyris Diamond
TG analyzer under air atmosphere with a heating ramp of 5 K/min.
Solid-state 13C (100.5 MHz) and 29Si (79.4 MHz) CP MAS NMR were
obtained on a Bruker DRX-400 spectrometer (CP times: 2 ms,
a
l
a
analysis (%): C 9.07, H 1.66, N 1.29, S 2.21. SBET: 516.4 m2/g, dpore
7.85 nm, Vpore
0.72 cm3/g. 29Si MAS NMR (79.4 MHz): T1
:
:
(d = À48.3 ppm), T2 (d = À58.5 ppm), T3 (d = À65.6 ppm). 13C CP/
MAS NMR (161.9 MHz): 137.5, 123.7 (CH of imidazolium), 119.5
(CH of Ph), 97.9 (C of Cp ring), 69.8–67.6 (ANCHPhA), 56.5 (AOCH2-
CH3), 47.3 (ANCH2CH2CH2SA), 37.2 (ANCH2CH2CH2SiA), 21.9
(AOCH2CH3), 16.4 (ANCH2CH2CH2SA), 12.3 (CpCH3), 8.2–0.5
(ACH2Si) ppm.
2.2.3. Preparation of the heterogeneous catalyst 6
6 was prepared according to the general procedure for 5 using
(S,S)-methylsulfonyl-1,2-diphenylethylenediamine instead of 1,2-
bis(triethoxysilyl)ethylane-(S,S)-pentafluorophenylsulfonyl-1,2-
diphenylethylenediamine. The solid was dried under reduced pres-
sure overnight to afford catalyst 6 as a light-yellow powder. ICP
analysis showed that the Ir loading was 48.35 mg (0.25 mmol)
per gram of catalyst and the residual Ag amount was 9.83 mg
(0.091 mmol) per gram of catalyst. IR (KBr) cmÀ1: 3455.1 (s),
2926.1 (w), 2895.9 (w), 1635.1 (m), 1411.4 (w), 1268.7 (m),
1156.9 (s), 1031.7 (s), 919.9 (m), 788.1 (m), 706.3 (m), 452.6 (m).
spinning speed 5 KHz, pulse lengths: 4 ls). Elemental analysis
was performed with a Carlo Erba 1106 Elemental Analyzer.
2.2. Catalyst preparation
2.2.1. Preparation of HO3S-PMO (3)
In a typical synthesis, 2.0 g of structure-directing agent, pluron-
ic P123, was completely dissolved in a mixture of 80 mL of hydro-
chloric acid (0.2 N) and 6.0 g of KCl. The mixture was stirred at
room temperature for 1.0 h. Subsequently, 3.22 g (9.10 mmol) of
the silica precursor 1,2-bis(triethoxysilyl)ethane was added at
40 °C. After a pre-hydrolysis period of 40 min, 0.44 g (0.90 mmol)
of 3-(3-sulfopropyl)-1-(3-(triethoxysilyl)propyl)-1H-imidazol-3-
ium hydrosulfate (1) was added. The reaction mixture was stirred
at 40 °C for 24 h and then aged at 100 °C for 24 h. The resulting
solid was filtered, rinsed with excess ethanol, and then dried over-
night on a filter. The surfactant template was removed by refluxing
in acidic ethanol (400 mL per gram) for 24 h. The solid was filtered,
rinsed with ethanol again, and then dried at 60 °C under reduced
pressure overnight to afford HO3S-PMO (3) (1.82 g) in the form
of a white powder. IR (KBr) cmÀ1: 3434.7 (s), 2983.0 (w), 2896.2
(w), 1634.6 (m), 1412.7 (w), 1261.7 (m), 1158.8 (s), 1031.8 (s),
904.9 (m), 808.7 (m), 697.8 (m). Elemental analysis (%): C 15.82,
Elemental analysis (%): C 7.71, H 1.72, N 1.40, S 2.41; SBET
:
524.2 m2/g, dpore 0.75 cm3/g. 29Si MAS NMR
:
7.92 nm, Vpore
:
(79.4 MHz): T1 (d = À49.4 ppm), T2 (d = À58.4 ppm), T3
(d = À65.1 ppm). 13C CP/MAS NMR (161.9 MHz): 137.6, 123.9 (CH
of imidazolium), 119.2 (CH of Ph), 97.8 (C of Cp ring), 69.9–67.4
(ANCHPh), 56.5 (AOCH2CH3), 47.3 (ANCH2CH2CH2SA, CH3SO2A),
37.3 (ANCH2CH2CH2SiA), 22.0 (AOCH2CH3), 16.4 (ANCH2CH2CH2-
SA), 12.2 (CpCH3), 8.3–0.5 (ACH2Si) ppm.
2.3. Catalytic experiments
A typical procedure was as follows. For enantioselective reduc-
tion of
(8.70 mg, 2.0
(0.80 mmol), and an aqueous solution of formic acid (5.0 equiv.
a
-cyanoacetophenones or
a-nitroacetophenones, catalyst 5
lmol of Ir, based on ICP analysis), acetophenones

72
B. Deng et al. / Journal of Catalysis 320 (2014) 70–76
1.0 M formate solution, 0.2 M overall concentration, for X = CN,
pH = 3.5; for X = NO2, pH = 2.0) were added sequentially to a
10.0 mL round–bottom flask. The mixture was then stirred at room
temperature (25 °C) for 5–20 h. [For enantioselective reduction of b-
100
80
60
40
20
ketoesters, catalyst 6 (8.0 mg, 2.0 lmol of Ir, based on ICP analysis),
b-ketoesters (0.80 mmol), and an aqueous sodium formate solution
(5.0 equiv. 1.0 M formate solution, 0.2 M overall concentration,
pH = 8.0) were added to a 10.0 mL roundbottom flask in turn. The
mixture was then stirred at 4 °C for 6–8 h.] During this period,
the reaction was monitored constantly by TLC. After the comple-
tion of the reaction, the catalyst was separated by centrifugation
(10,000 rpm) for the recycling experiment. The aqueous solution
was extracted with ethyl ether (3 Â 3.0 mL). The combined ethyl
ether extracts were washed with brine twice and then dehydrated
with Na2SO4. After evaporation of ethyl ether, the residue was
purified by silica gel flash column chromatography to afford the
desired product. The conversion was calculated through the exter-
nal standard method, and the ee value was determined by a HPLC
analysis using a UV–Vis detector and a Daicel OJ-H chiralcel col-
Catalyst 5
3
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 1. FT-IR spectra of HO3S-PMO (3) and catalyst 5.
weak bands between 3100–2800 cmÀ1 were assigned to asymmet-
umn (
U
0.46 Â 25 cm).
ric and symmetric stretching vibrations of CAH bonds. The peaks
indicative of
were difficult to be distinguished because of the overlapping by
the absorbance from (SiAO) [33]. The bands between 1610–
t
(SiAC) should appear at 1100 cmÀ1, however, they
3. Results and discussions
t
3.1. Synthesis and characterization of catalyst 5
1450 cmÀ1 were attributed to breathing vibrations of C@C bonds
in the aromatic ring [32]. These peaks increased consistently in cat-
alyst 5 relatively to that in the spectrum of 3, implying that (S,S)-
pentafluorophenylsulfonyl-1,2-diphenylethylenediamine (4) par-
ticipated in the coordination of Ir centers. All these observations
demonstrated the successful incorporation of chiral Cp⁄IrPFPSD-
PEN complexes within the periodic mesoporous organosilica.
Incorporation of well-defined single-site active iridium centers
within the organosilicate network of 5 could be proven by solid-
state 13C cross-polarization (CP)/magic angle spinning (MAS)
NMR spectroscopy. As shown in Fig. 2, both 3 and 5 produced
strong, characteristic carbon signals of SiCH2CH2Si groups at
ꢀ5 ppm, corresponding to ethylene-bridged organosilica embed-
ded within their organosilicate networks. Carbon atoms of NCH
groups of the TsDPEN moiety could be observed clearly at
ꢀ68 ppm in their spectra. Peaks at ꢀ98 and ꢀ12 ppm in the spec-
Imidazolium-based, chiral Cp⁄IrPFPSDPEN-functionalized peri-
odic mesoporous organosilica, abbreviated as Cp⁄IrPFPSDPEN-
PMO (5), (Cp⁄IrPFPSDPEN: [25–27] Cp⁄ = pentamethylcyclopenta-
diene, PFPSDPEN = (S,S)-pentafluorophenylsulfonyl-1,2-diphenyle-
thylenediamine (4)) was prepared as outlined in Scheme 1. First,
3-(3-sulfopropyl)-1-(3-(triethoxysilyl)propyl)-1H-imidazol-3-ium
hydrosulfate (1) was obtained through the reaction of 1-(3-(trieth-
oxysilyl)propyl)-1H-imidazole and 1,3-propane-sultone according
to a reported method [28]. Co-condensation of 1 and 1,2-bis(trieth-
oxysilyl)ethylane (2) using P123 as a structure-directing template
afforded HO3S-PMO (3) as a white powder. Finally, the continuous
ion exchanges with Ag2O [29] and (Cp⁄IrCl2)2 followed by complex-
ation of (4) [30] led to the crude heterogeneous catalyst 5 in which
the potential formation of imidazolyl-Ag or imidazolyl-Ir com-
plexes had not been observed [29,31]. This was subjected to Soxh-
let extraction to clear its nanochannels and to obtain its pure form,
which was a light-yellow powder.
Fig. 1 shows the FT-IR spectra of the HO3S-PMO (3) and catalyst
5. Generally, both 3 and 5 exhibited the characteristic bands of the
PMO-type materials around 3430, 1635 and 1031 cmÀ1 for
i b
2
l
a
e
j
a
Si
f
SO3
f
N
Ir
OH2
t(OAH), d(OAH) and t(SiAO), respectively [32]. The relatively
Si CH2 CH2
SO2C6F5
Si
O
N
c
c
2
n
N
H2N
OgCH CH
2d
m
3
h
k
H
5
Catalyst
Ph i
Ph
a
3
f
b
Catalyst 5
c
j
d
l
i
e
k
g
h
180
160
140
120
100
80
60
40
20
0
-20
ppm
Scheme 1. Preparation of the heterogeneous catalyst 5.
Fig. 2. 13C CP MAS NMR spectra of HO3S-PMO (3) and catalyst 5.

B. Deng et al. / Journal of Catalysis 320 (2014) 70–76
73
trum of 5 are ascribed to the carbon atoms of the Cp ring and to the
1400
1200
1000
800
Ir: 4f7/2
Catalyst 5
carbon atoms of the CH3 groups attached to Cp, respectively. These
peaks are absent in the spectrum of 3, suggesting the formation of
the Cp⁄IrPFPSDPEN complex. Chemical shifts of 5 were similar to
those of its homogeneous counterpart (Cp⁄IrPFPSDPEN) [25], dem-
onstrating that they had the same well-defined single-site active
species. In addition, carbon signals of imidazolium and of propyl
moiety could also be observed (marked in the spectra). Peaks at
ꢀ22 and ꢀ57 ppm typically present in the 13C CP/MAS spectra
are attributed to the unhydrolyzed ethoxy groups (AOCH2CH3)
[34].
62.46
Cp*IrPFPSDPEN
62.48
600
400
The organosilicate network and the composition of Cp⁄IrPFPSD-
PEN active centers in 5 could be confirmed by solid-state 29Si MAS
NMR spectroscopy. As shown in Fig. 3, both 3 and 5 produced one
group of exclusive T signals derived from organosilica, suggesting
that all Si species were covalently attached to carbon atoms [6].
Typical isomer shift values of 5 were similar to values reported
in the literature [35] (À48.5, À58.5, and À67.5 ppm for T1, T2,
and T3 of [R(HO)2SiOSi], [R(HO)Si(OSi)2], and [RSi(OSi)3], respec-
tively). Notably, strong T signals at À58 and À65 ppm in the spec-
trum of 5 are attributed to T2(R–Si(OSi)2(OH)] and T3(R–Si(OSi)3),
respectively (R = Cp⁄IrPFPSDPEN functionalities or ethylene-
bridged groups). These observations demonstrate that both 3 and
5 possessed organosilicate networks with R-Si(OSi)2(OH) and R-
Si(OSi)3 species in their main organosilicate networks. Further-
more, the absence of signals for Q-series from À90 to À120 ppm
indicates that the carbon–silicon bond was not cleaved during
the hydrolysis–condensation process.
To investigate the electronic state of the iridium center, X-ray
photoelectron spectroscopy (XPS) of 5 and its homogeneous Cp⁄-
IrPFPSDPEN was carried out. As shown in Fig. 4, 5 had Ir 4f7/2 bind-
ing energy similar to that of Cp⁄IrPFPSDPEN (62.48 eV vs 62.46 eV),
but both Ir 4f7/2 binding energies were obviously different from
that of their parent (Cp⁄IrCl2)2 complex (61.81 eV) (see SI in
Fig. S1). This difference indicates the formation of single-site Cp⁄-
IrPFPSDPEN active centers within the organosilicate network of 5.
These results are verified by 13C CP/MAS NMR spectra. The active
centers play a key role in the enantioselective performance, as dis-
cussed in the next section.
200
56
58
60
62
64
66
68
70
72
74
Electron Binding Energy (eV)
Fig. 4. XPS spectra of Cp⁄IrPFPSDPEN and catalyst 5.
100
80
60
40
20
0
Table: Structural parameters
Material SBET (m2/g) DP(nm) VP (cm3/g)
3
5
740.0
516.4
8.02
7.85
0.90
0.72
3
Catalyst 5
1
2
3
4
2 Theta/degree
Fig. 5. Small-angle powder XRD patterns of HO3S-PMO (3) and catalyst 5.
The ordered mesostructure and well-defined pore arrangement
of 5 were further investigated by using X-ray diffraction (XRD),
transmission electron microscopy (TEM), and nitrogen adsorp-
tion–desorption measurements. The small-angle XRD patterns
(Fig. 5) reveal that both 3 and 5 produced an intense d100 diffrac-
hexagonal arrangements, in which the SEM image with a chemical
mapping of 5 had the uniform distribution of Ir (see SI in Fig. S2).
The nitrogen adsorption–desorption isotherm of 5 (Fig. 7) is a typ-
ical type IV isotherm with an H1 hysteresis loop and a visible step
at P/P0 = 0.50–0.90, corresponding to capillary condensation of
nitrogen in mesopores. Structural parameters of 3 and 5 are listed
in a table inset in Fig. 5.
Above characterizations and analyses suggest that the imidazo-
lium-based, chiral organoiridium-functionalized PMO containing
well-defined single-site Cp⁄IrPFPSDPEN active species within its
ordered dimensional-hexagonal nanostructure could be readily
constructed through the co-condensation strategy.
tion peak along with two similar weak diffraction peaks (d110
,
d200), suggesting that the dimensional-hexagonal pore structure
(p6mm) observed in general PMO materials could be preserved
after co-condensation [18,19]. The TEM images (Fig. 6) further con-
firm that 5 had a highly ordered mesostructure with dimensional-
T3
T2
3
Catalyst 5
3.2. Catalytic property of the heterogeneous catalyst 5
3.2.1. Catalytic performance
The chiral N-sulfonylated diamine-based organometallic com-
plexes, a type of highly enantioselective catalysts, have been used
extensively in various studies on asymmetric transfer hydrogena-
tion [25–27,36]. In the present study, we first examined its cata-
T1
lytic activity and enantioselectivity in the reduction of
a-
cyanoacetophenone in aqueous medium. Reaction with the hydro-
gen source HCO2H and 0.25% mol PMO as a catalyst was investi-
gated according to a reported method [25]. Enantioselective
-20
-40
-60
-80
-100
-120
ppm
reduction of benzoylacetonitrile catalyzed by
5 gave (S)-3-
Fig. 3. 29Si CP MAS NMR spectra of HO3S-PMO (3) and catalyst 5.
hydroxy-3-phenylpropanenitrile with >99% conversion and 97%

74
B. Deng et al. / Journal of Catalysis 320 (2014) 70–76
Catalyst 5
Fig. 6. TEM images of HO3S-PMO (3) and catalyst 5 viewed along [100] and [001] directions.
with Cp⁄IrPFPSDPEN [25]. For example, we found that the enantio-
600
500
400
300
200
100
selective reduction of benzoylacetonitrile catalyzed by 5 could be
completed within 5 h, whereas the reaction catalyzed by its inorga-
nosilicate analog and by Cp⁄IrPFPSDPEN required 12 h [22] and
24 h, respectively. The drastically enhanced reaction rates attained
with 5 are due to the potential imidazolium-functionality and high
hydrophobicity of ethylene-bridged PMO. To confirm this judg-
ment, the kinetic profile of the enantioselective reduction of ben-
zoylacetonitrile catalyzed by 5, by its inorganosilicate analog, and
by Cp⁄IrPFPSDPEN were investigated. The initial activity of 5 was
evidently higher than that of its inorganosilicate analog, and even
higher than that of Cp⁄IrPFPSDPEN; the initial TOF values were
122.0, 60.0 and 39.0 mol/(mol h), respectively (Fig. 8). Notably,
the reaction rate with 5 was twice that with its inorganosilicate-
supported analog and thrice that with Cp⁄IrPFPSDPEN. These
results suggest that the enhanced rate of asymmetric organic
4
8
12
16
20
Pore diameter (nm)
3
Catalyst 5
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Table 1
Enantioselective reduction of a-cyano and a
-nitroacetophenones.a
Fig. 7. Nitrogen adsorption–desorption isotherms of HO3S-PMO (3) and catalyst 5.
ee value. Such an ee value seems to be a small increase relative to
that of Cp⁄IrPFPSDPEN (entry 1 vs entry 1 in brackets, Table 1). In
addition, the asymmetric reaction could be run at much higher
substrate-to-catalyst mole ratio without apparently affecting its
ee value, as exemplified by the enantioselective reduction of ben-
zoylacetonitrile at substrate-to-catalyst mole ratio of 600 (entry
2, Table 1).
Entry
Ar, X
Time (h)
Conv. (%)b
Ee (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Ph, CN
Ph, CN
Ph, CN
5 (24)
20
5
5
5
5
5
10
10
5
>99 (99)
93
95
97 (94)c
96d
94e
96
94
96
95
98
97
98
Because of its high catalytic performance, 5 was further investi-
gated by using it in the enantioselective reductions of a-cyanoace-
4-FPh, CN
3-ClPh, CN
4-MePh, CN
3-MeOPh, CN
2-furyl, CN
2-thiophenyl, CN
Ph, NO2
4-FPh, NO2
4-ClPh, NO2
2-ClPh, NO2
4-MePh, NO2
4-MeOPh, NO2
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
tophenones with various substituents on the Ar moiety. As shown
in Table 1, high conversions, no side products, and high enantiose-
lectivities were obtained for all tested substrates under the same
reaction conditions. The electronic properties of substituents at
the Ar moiety of acetophenones did not affect enantioselectivities,
that is, various electron-withdrawing and -donating substituents
of the Ar moiety led to the same efficiency (entries 4–9). The enan-
tioselective reduction was also suitable for the asymmetric transfer
5
5
5
5
95
94
92
97
5
95
hydrogenation of
sions, no side products, and high enantioselectivities could also
be obtained for a series of
(entries 10–15).
a-nitroacetophenones. Similarly, high conver-
a
Reaction conditions: catalyst 5 (8.70 mg, 2.0
lmol of Ir based on the ICP anal-
ysis), -cyanoacetophenones -nitroacetophenones) (0.80 mmol) and the
a
(a
a-nitro-substituted acetophenones
aqueous solution of formic acid (5.0 equiv, 1.0 M formate solution, 0.2 M overall
concentration, for X = CN, pH = 3.5; for X = NO2, pH = 2.0), reaction temperature
(25 °C) and reaction time (5–20 h).
Determined by chiral HPLC analysis (see SI in Figs. S3 and S7).
Data in the bracket were obtained using the homogenous Cp⁄IrPFPSDPEN as a
catalyst reported in the literature [7].
Data were obtained using catalyst 5 at substrate-to-catalyst mole ratio of 600.
Data were obtained using the mixed HO3S-PMO (3) plus its homogeneous
Cp⁄IrPFPSDPEN as a catalyst.
b
c
3.2.2. Investigation of factors affecting catalytic performance
It is noteworthy that the asymmetric reactions catalyzed by 5
have rates higher than that attained with a previously reported
inorganosilica-supported Cp⁄IrPFPSDPEN-functionalized heteroge-
neous catalyst [22], and even markedly higher than that attained
d
e

B. Deng et al. / Journal of Catalysis 320 (2014) 70–76
75
obtained with Cp⁄IrPFPSDPEN reveals that the reaction catalyzed
by the catalyst mixture retained its original, homogenous enanti-
oselectivity [22]. However, the ee value is low relative to that
obtained with 5 (94% ee vs 97% ee), suggesting that the enhanced
ee value with 5 was originated from the imidazolium-based chiral
Cp⁄IrPFPSDPEN-PMO material itself. Indirect evidence supporting
this hypothesis came from an XPS investigation discussed above,
in which both the similar Ir 4f7/2 binding energy indicates that
the active iridium center in 5 had the same coordinated environ-
ment as that of Cp⁄IrPFPSDPEN. Thus, the enhanced enantioselec-
tivity could be ascribed to the well-defined single-site active
iridium species with the ethylene-bridged organosilicate network
proved by above 13C NMR.
100
80
60
40
20
0
OH
O
CN
CN
Catalyst
Cp*IrPFPSDPEN)
Inorganosilicate analog
Catalyst 5
0
2
4
6
8
10 12 14 16 18 20 22 24
Reaction time (h)
3.2.3. Catalyst recycling and reuse
Another important consideration in the design of a heteroge-
neous catalyst is the ease of separation by simple centrifugation
and the ability of the catalyst to retain its catalytic activity and
enantioselectivity after multiple cycles of use. As shown in Table 2,
the heterogeneous catalyst 5 was recovered easily and reused
repeatedly in eight consecutive reactions for enantioselective
reduction of benzoylacetonitrile as a model reaction. At eighth
recycle, catalyst 5 still afforded 99.0% conversion and 94.5% ee
(see SI in Fig. S4). ICP analysis showed that the amount of Ir at
8th cycle was 42.30 mg per gram of catalyst and only 4.2% of the
Ir content was lost. This result should be ascribed the fact that
the uniformly distributed counterions associated with the imi-
dazolium and sulfonates within the wall of nanopores could avoid
Fig. 8. Comparison of the asymmetric transfer hydrogenations of benzoylacetonit-
rile catalyzed by 5, by its inorganosilicate analog, and by its homogeneous
Cp⁄IrPFPSDPEN. Reactions were carried out at substrate-to-catalyst mole ratio of
400.
transformation by 5 is due to the combination of the imidazolium-
functionality and the high hydrophobicity of ethylene-bridged
organosilicate.
Interestingly, asymmetric reactions catalyzed by 5 seem to be a
small increase in enantioselectivity for most tested substrates, rel-
ative to those with its homogeneous Cp⁄IrPFPSDPEN [25]. For
example, enantioselective reduction of benzoylacetonitrile cata-
lyzed by 5 gave 97% ee of the target products, which is higher than
94% ee obtained with its homogeneous Cp⁄IrPFPSDPEN. This differ-
ence indicates that the incorporation of chiral Cp⁄IrPFPSDPEN func-
tionality within the imidazolium-based PMO could result in
slightly enhanced enantioselectivity. To obtain deeper insight into
the effect, a parallel enantioselective reduction of benzoylacetonit-
rile catalyzed by a mixture of 3 plus its homogeneous Cp⁄IrPFPSD-
PEN was investigated. The result shows that the asymmetric
reaction gave (S)-3-hydroxy-3-phenylpropanenitrile with 95% con-
version and 94% ee (entry 3, Table 1). The same ee value as that
efficiently the leaching of Ir, thereby resulting in
recyclability.
a high
3.2.4. Expansion of enantioselective reduction
Furthermore, as an approach of direct postcoordination of chiral
ligands, the immobilized strategy could also be used to easily con-
struct other heterogeneous catalysts. As shown in Scheme 2, a sim-
ilar imidazolium-based chiral Cp⁄IrMsDPEN-functionalized PMO
(Cp⁄IrMsDPEN: [37,38] MsDPEN = methylsulfonyl-1,2-diphenyle-
thylenediamine), denoted as Cp⁄IrMsDPEN-PMO (6), could be read-
ily constructed by substitution of (S,S)-PFPSDPEN with (S,S)-
Table 2
Reusability of catalysts 5–6 for asymmetric reactions.
Catalyst
Run time
1
2
3
4
5
6
7
8
9
5
% Conv.a
% eea
>99.9
97.1
>99.9
97.2
99.5
96.1
99.7
97.2
99.5
95.8
99.5
97.1
99.5
94.4
99.4
96.8
99.3
94.8
99.4
96.4
99.3
94.8
99.3
96.4
99.1
94.6
99.3
96.3
99.0
94.5
99.1
96.0
88.2
94.4
87.6
94.8
6
% Conv.b
% eeb
a
Reaction conditions: Catalyst 5 (87.0 mg, 20.0
lmol of Ir based on the ICP analysis), benzoylacetonitrile (8.0 mmol), the aqueous solution of formic acid (5.0 equiv, 1.0 M
formate solution, 0.2 M overall concentration, pH = 3.5), reaction temperature (25 °C), reaction time (5 h).
b
Reaction conditions: catalyst 6 (80.0 mg, 20.0
lmol of Ir based on the ICP analysis), ethyl benzoylacetate (8.0 mmol), aqueous sodium formate solution (5.0 equiv. 1.0 M
formate solution, 0.2 M overall concentration, pH = 8.0), reaction temperature (4 °C) and reaction time (6 h).
Scheme 2. Structure of the heterogeneous catalyst 6.

76
B. Deng et al. / Journal of Catalysis 320 (2014) 70–76
Table 3
Acknowledgments
Enantioselective reduction of b-ketoesters.a
We are grateful to Shanghai Sciences and Technologies Develop-
ment Fund (12nm0500500 and 13ZR1458700), CSIRT (IRT1269)
and Shanghai Municipal Education Commission (12ZZ135,
14YZ074 and SK201329) for financial supports.
Conv. (%)b
Ee (%)b
Appendix A. Supplementary material
Entry
R
Time (h)
1
2
3
4
5
6
7
8
9
H
4-F
3-Cl
2-Cl
4-Br
4-I
4-Me
4-OMe
4-NO2
4-CF3
2-furyl
6 (24)
>99 (96)
>99
>99
>99
>99
>99
>99
>99
>99
97 (96)
95
94
60
98
94
97
97
95
Experimental procedure and analytical data for obtained chiral
products. This material is available free of charge via editor office.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2014.09.019.
6
6
6
6
6
6
6
6
6
8
References
[1] N. Mizoshita, T. Taniab, S. Inagaki, Chem. Soc. Rev. 40 (2011) 789.
[2] F. Hoffmann, M. Fröba, Chem. Soc. Rev. 40 (2011) 608.
[3] F. Kleitz, Handbook of Asymmetric Heterogeneous Catalysis, Wiley-VCH,
Weinheim, 2008. pp. 178.
10
11
>99
>99
96
97
a
Reaction conditions: catalyst 6 (8.0 mg, 2.0 lmol of Ir based on the ICP analy-
[4] C.E. Song, in: K.L. Ding, Y. Uozumi (Eds.), Handbook of Asymmetric
Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2009, p. 25.
[5] M.E. Davis, Nature 417 (2002) 813.
[6] S. Inagaki, S. Guan, T. Ohsuna, O. Terasaki, Nature 416 (2002) 304.
[7] J. Dupont, R.F. de Souza, P.A.Z. Suarez, Chem. Rev. 102 (2002) 3667.
[8] B. Karimi, M. Gholinejad, M. Khorasani, Chem. Commun. 48 (2012) 8961.
[9] B. Karimi, D. Elhamifar, O. Yari, M. Khorasani, H. Vali, J.H. Clark, A.J. Hunt,
Chem. Eur. J. 18 (2012) 13520.
sis), b-ketoesters (0.80 mmol) and the aqueous sodium formate solution (5.0
equiv. 1.0 M formate solution, 0.2 M overall concentration, pH = 8.0), reaction
temperature (4 °C), reaction time (6–8 h).
b
Determined by chiral HPLC analysis (see SI in Figs. S5 and S7).
[10] P. Borah, X. Ma, K.T. Nguyen, Y. Zhao, Angew. Chem. Int. Ed. 51 (2012) 7756.
[11] M. Waki, N. Mizoshita, T. Tani, S. Inagaki, Angew. Chem. Int. Ed. 50 (2011)
11667.
[12] M. Guan, W. Wang, E.J. Henderson, Ö. Dag, C. Kübel, V.S.K. Chakravadhanula, J.
Rinck, I.L. Moudrakovski, J. Thomson, J. McDowell, A.K. Powell, H. Zhang, G.A.
Ozin, J. Am. Chem. Soc. 134 (2012) 8439.
[13] M. Mandal, Mi. Kruk, Chem. Mater. 24 (2012) 123.
[14] A. Kuschel, S. Polarz, J. Am. Chem. Soc. 132 (2010) 6558.
[15] X. Wu, T. Blackburn, J.D. Webb, A.E. Garcia-Bennett, C.M. Crudden, Angew.
Chem. Int. Ed. 50 (2011) 8095.
[16] X. Liu, P.Y. Wang, L. Zhang, J. Yang, C. Li, Q.H. Yang, Chem. Eur. J. 16 (2010)
12727.
[17] R.A. Garcia, R. van Grieken, J. Iglesias, V. Morales, N. Villajos, J. Catal. 274
(2010) 221.
[18] K. Liu, R. Jin, T. Cheng, X. Xu, F. Gao, G. Liu, H. Li, Chem. Eur. J. 18 (2012) 15546.
[19] R. Liu, R. Jin, L. Kong, J. Wang, C. Chen, T. Cheng, G. Liu, Chem. Asian. J. 8 (2013)
3108.
MsDPEN through the same procedure. As shown in Table 3, 6 also
displayed high catalytic activity and high enantioselectivity in the
reduction of b-ketoesters in aqueous medium. Similarly, enhanced
reaction ratio and enantioselectivity could be obtained in this case.
As expected, the heterogeneous catalyst 6 could also be recovered
easily and reused repeatedly in the enantioselective reduction of
ethyl benzoylacetate as a model reaction (Table 2). The recycling
catalyst 6 still afforded 99.1% conversion and 96.0% ee at eighth
recycle (see SI in Fig. S6). This result suggests that the immobiliz-
ated strategy described here could be a potential way for anchoring
various chiral ligands to construct imidazolium-based, chiral
organometal-functionalized periodic mesoporous organosilica.
[20] W. Xiao, R. Jin, T. Cheng, D. Xia, H. Yao, F. Gao, B. Deng, G. Liu, Chem. Commun.
48 (2012) 11898.
[21] D. Xia, T. Cheng, W. Xiao, K. Liu, Z. Wang, G. Liu, H. Li, W. Wang, ChemCatChem
5 (2013) 1784.
4. Conclusions
In conclusions, using a co-condensation method, we incorpo-
rated the imidazolium functionality within an ethylene-bridged
PMO. Utilizing a direct postcoordination of chiral ligands, we devel-
oped two chiral organoiridium-functionalized heterogeneous cata-
[22] B. Deng, T. Cheng, M. Wu, J. Wang, G. Liu, ChemCatChem 5 (2013) 2856.
[23] J. Long, G. Liu, T. Cheng, H. Yao, Q. Qian, J. Zhuang, F. Gao, H. Li, J. Catal. 298
(2013) 41.
[24] R. Liu, T. Cheng, L. Kong, C. Chen, G. Liu, H. Li, J. Catal. 307 (2013) 55.
[25] O. Soltani, M.A. Ariger, H. Vázquez-Villa, E.M. Carreira, Org. Lett. 12 (2010)
2893.
[26] H. Vázquez-Villa, S. Reber, M.A. Ariger, E.M. Carreira, Angew. Chem. Int. Ed. 50
(2011) 8979.
[27] R. ter Halle, E. Schulz, M. Lemaire, Synlett (1997) 1257.
[28] J.M. Miao, H. Wan, Y.B. Shao, G.F. Guan, B. Xu, J. Mol. Catal. A: Chem. 348
(2011) 77.
lysts. The chiral PFPSDPEN-based heterogeneous catalyst
5
displayed catalytic activity and enantioselectivity higher than those
of its homogeneous counterpart Cp⁄IrPFPSDPEN in the enantiose-
lective reduction of
a-cyanoacetophenones and a-nitroacetophe-
nones in aqueous medium. As presented in this study, the
imidazolium-functionality and the hydrophobic PMO could
enhance the catalytic performance in aqueous asymmetric reac-
tions. In addition, the heterogeneous catalyst 5 could be recovered
and reused for at least eight times without loss of its catalytic activ-
ity. The immobilization approach could be used to easily construct
another chiral, methylsulfonyl-1,2-diphenylethylenediamine-
based organoiridium-functionalized-PMO (6) and to realize highly
efficient enantioselective reduction of b-ketoesters in aqueous
medium.
[29] S. Hiraoka, M. Shiro, M. Shionoya, J. Am. Chem. Soc. 126 (2004) 1214.
[30] S. Ogo, N. Makihara, Y. Watanabe, Organometallics 18 (1999) 5470.
[31] T.K. Maishal, J. Alauzun, J. Basset, C. Copéret, R.J.P. Corriu, E. Jeanneau, A.
Mehdi, C. Reyé, L. Veyre, C. Thieuleux, Angew. Chem. Int. Ed. 47 (2008) 8654.
[32] A.S.M. Chong, X.S. Zhao, J. Phys. Chem. B 107 (2003) 12650.
[33] H. Huang, R. Yang, D. Chinn, C.J. Munson, Ind. Eng. Chem. Res. 42 (2003) 2427.
[34] P.F.W. Simon, R. Ulrich, H.W. Spiess, U. Wiesner, Chem. Mater. 13 (2001) 3464.
}
}
}
[35] O. Krocher, O.A. Koppel, M. Froba, A. Baiker, J. Catal. 178 (1998) 284.
[36] R. Malacea, R. Poli, E. Manoury, Coordin. Chem. Rev. 254 (2010) 729.
[37] M.A. Ariger, E.M. Carreira, Org. Lett. 14 (2012) 4522.
[38] J.H. Xie, X.Y. Liu, X.H. Yang, J.B. Xie, L.X. Wang, Q.L. Zhou, Angew. Chem. Int. Ed.
51 (2012) 201.