Preparation of surface molecularly imprinted Ru-complex catalysts for asymmetric transfer hydrogenation in water media

Article abstract of DOI:10.1039/c0dt00950d

Molecularly imprinted Ru-complex catalysts acting in water were prepared on a SiO₂ surface by molecular imprinting of a SiO₂- supported Ru-complex using organic polymers as surface matrix overlayers. (R)-1-(o-fluorophenyl)ethanol, which is one of the hydrogenated products of o-fluoroacetophenone, was imprinted on the supported Ru-complex as a template, and an active Ru-complex with a shape-selective reaction space (molecularly imprinted cavity) was prepared inside the wall of the hydrophobic organic polymer matrix overlayers. Structures of the SiO₂-supported and molecularly imprinted Ru catalysts were characterized by means of solid-state NMR, XPS, XRF, ICP, UV/vis, XAFS, TGA, and SEM. The molecularly imprinted Ru catalysts exhibited fine shape selectivity and enantioselectivity for the asymmetric transfer hydrogenation of o-fluoroacetophenone and its derivatives.

Full text of DOI:10.1039/c0dt00950d

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Preparation of surface molecularly imprinted Ru-complex catalysts for asymmetric transfer
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Zhihuan Weng, Satoshi Muratsugu, Nozomu Ishiguro, Shin-ichi Ohkoshi and Mizuki Tada
Dalton Trans., 2011, DOI: 10.1039/C0DT00950D
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PAPER
Preparation of surface molecularly imprinted Ru-complex catalysts for
asymmetric transfer hydrogenation in water media†
Zhihuan Weng,a,b Satoshi Muratsugu,a,c Nozomu Ishiguro,a,d Shin-ichi Ohkoshid and Mizuki Tada*a,c
Received 3rd August 2011, Accepted 11th November 2011
DOI: 10.1039/c0dt00950d
Molecularly imprinted Ru-complex catalysts acting in water were prepared on a SiO2 surface by
molecular imprinting of a SiO2-supported Ru-complex using organic polymers as surface matrix
overlayers. (R)-1-(o-fluorophenyl)ethanol, which is one of the hydrogenated products of
o-fluoroacetophenone, was imprinted on the supported Ru-complex as a template, and an active
Ru-complex with a shape-selective reaction space (molecularly imprinted cavity) was prepared inside
the wall of the hydrophobic organic polymer matrix overlayers. Structures of the SiO2-supported and
molecularly imprinted Ru catalysts were characterized by means of solid-state NMR, XPS, XRF, ICP,
UV/vis, XAFS, TGA, and SEM. The molecularly imprinted Ru catalysts exhibited fine shape
selectivity and enantioselectivity for the asymmetric transfer hydrogenation of o-fluoroacetophenone
and its derivatives.
Organic or inorganic monomer is polymerized in the presence
Introduction
of a template, and the subsequent extraction of the template
Many useful chemicals have been artificially produced by hetero-
from prepared polymer matrices produces a molecularly imprinted
geneous catalysts, and the construction of efficient heterogeneous
cavity with a similar shape to the template molecule. In order to
systems for selective catalysis is essential for further developments
apply molecular imprinting for the preparation of a shape selective
in novel chemical processes. Immobilization of a metal complex
catalyst, a molecularly imprinted cavity should be prepared close
on a support surface is still the state of the art in an assimilative
to a catalytically active site so that molecular imprinting of a metal
field between homogeneous and heterogeneous catalyses,1–7 and
it has currently received much attention in practice for a variety
of catalytic chemical syntheses. Metal-complex precursors can be
directly attached on oxide surfaces in a controllable manner8–13
and also grafted with organic branches on oxide surfaces.14–19
Zeolite,20,21 MCM,22 and SBA,23–25 which are porous oxide sup-
ports, and ionic liquid modified SiO226,27 can be utilized as supports
of metal complexes. Attaching of a metal complex on an oxide
surface provides not only the improvement of its stability under
catalytic reaction conditions but also unique activity derived from
its coordination structure on the oxide surface.
complex, whose ligand is utilized as a template, is practical.57–74
In 2002, we have reported molecularly imprinted Rh-dimer
and Rh-monomer catalysts supported on SiO2 for shape-selective
hydrogenation of alkenes.68–74 A P(OCH3)3 ligand coordinating to
a Rh dimer or a Rh monomer supported on SiO2 was used as
a template, and SiO2-matrix overlayers were stacked on the SiO2
surface by the hydrolysis polymerization of Si(OCH3)4. Behind
the template, shape-selective reaction space with a similar shape
to P(OCH3)3 was created on active Rh complexes and fine shape
selectivity was observed for alkene hydrogenation. Furthermore,
molecular imprinting of a supported metal complex provides not
only a shape-selective reaction space but also the stabilization
effect of an active metal complex by surface matrix overlayers.75,76
Herein, we report the design, preparation, characteriza-
tion, and catalytic performances of molecularly imprinted
Ru-complex catalysts on SiO2 for the asymmetric trans-
fer hydrogenation of o-fluoroacetophenone in water. (R)-1-(o-
Fluorophenyl)ethanol, which is one of the hydrogenated products
of o-fluoroacetophenone, was coordinated to a SiO2-supported
Ru-complex as a template, and hydrophobic organic polymer
matrices were stacked on the surface of SiO2 by the photopoly-
merization of ThreeBond 3026E. A molecularly imprinted Ru-
complex behind the template was active and stable for the
transfer hydrogenation of o-fluoroacetophenone in water and
exhibited high enantioselectivity and fine shape selectivity. To our
Molecular imprinting,28–33 which is proposed to be a technique
to create an imprinted cavity with a similar shape to a template
molecule, has been applied to chromatographic separation,34–37
chemical sensors,38–42 binding assays,43–45 and selective catalysis.46–56
aInstitute for Molecular Science, Okazaki, 444-8585, Japan. E-mail:
mtada@ims.ac.jp; Fax: +81-564-55-2254; Tel: +81-564-55-7351
bDepartment of Applied Physics and Chemistry, The University of Electro-
Communication, Chofu, Tokyo, 182-8585, Japan
cThe Graduate University for Advanced Studies (SOKENDAI), Okazaki,
444-8585, Japan
dDepartment of Chemistry, Graduate School of Science, The University of
Tokyo, Hongo, Tokyo, 113-0033, Japan
† Electronic supplementary information (ESI) available: 1: quantitative
analysis of the coordination of the template, 2: Ru K-edge EXAFS, 3.
TGA, and 4: SEM. See DOI: 10.1039/c0dt00950d
2338 | Dalton Trans., 2011, 40, 2338–2347
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The Royal Society of Chemistry 2011
©

Scheme 1 Preparation of a molecularly imprinted Ru-complex catalyst on SiO2 for asymmetric transfer hydrogenation of o-fluoroacetophenone.
knowledge, this is the first example of a molecularly imprinted
Ru-complex catalyst supported on an oxide surface for a catalytic
reaction in water media.
7.0–7.4 (m, 14H; Ar–H). 13C NMR (100 MHz, CDCl3, Me4Si):
d 60.4, 63.1 (CH(C6H5)), 116.7, 126.2, 126.4, 127.0, 127.1, 127.4,
127.5, 128.3, 128.4, 135.5, 139.0, 139.2, 141.0, 141.3 (Ar–C and
CH CH2). Anal. Calcd for C22H22N2O2S: C, 69.81; H, 5.86; N,
7.40. Found: C, 69.73; H, 6.00; N, 7.33.
Experimental
Materials
Preparation of [(p-cymene)Ru{(1R,2R)-NH2CH(Ph)CH(Ph)-
NSO2C8H7}Cl] (B)63
All reagents commercially available were of research grade and
used without further purification, unless otherwise noted. All
procedures were carried out under N2 atmosphere. The prepa-
ration of the molecularly imprinted Ru catalyst is illustrated in
Scheme 1.
A solution of NaOEt in absolute methanol was added to
a solution of (1R,2R)-N-p-styrenesulfonyl-1,2-diphenylethylene
diamine (0.756 g, 2 mmol) in absolute CH2Cl2–MeOH (1 : 1;
15 mL). The resulting solution was slowly added to a solution
of [RuCl2(p-cymene)]2 (0.306 g, 1 mmol) in CH2Cl2 (20 mL). After
3 h, the solvent was evaporated in vacuo and the residue was stirred
with CH2Cl2–Et2O (2 : 1, 30 mL). After filtration, the solution was
reduced to a volume of 2 mL. After the addition of n-hexane
(15 mL), the solvent was evaporated under vacuum, resulting
Synthesis of (1R,2R)-N-p-styrenesulfonyl-1,2-diphenyl-
ethylenediamine77
1.0 g (4.7 mmol) (1R,2R)-1,2-diphenylethylenediamine was dis-
solved in a mixture of 15 mL of CH2Cl2 and 5 mL of Et3N. After
0.95 g (4.7 mmol) of p-styrenesulfonyl chloride in 10 mL of
CH2Cl2 was added at 273 K, the solution was stirred overnight
at 293 K. The solution was washed with 18 mL of a 5% NaOH
aqueous solution and dried with MgSO4 and then the solvent
was evaporated under vacuum. The obtained white residue was
dissolved in Et2O and 3 mL of a 37% HCl aqueous solution
was added. White precipitate was treated with 26 mL of a 5%
NaOH aqueous solution and then extracted with CH2Cl2 (3 ¥
40 mL). After drying with MgSO4 and evaporating the solvent
under vacuum, the product was obtained. Yield: 1.21 g (3.2 mmol,
68%). 1H NMR (400 MHz, CDCl3, Me4Si): d 4.15 (d, 1H,
CH(NH2)(C6H5)), 4.40 (d, 1H, CH(N–SO2)(C6H5)), 5.38 (d, 1H,
CH CH2), 5.79 (d, 1H, CH CH2), 6.66 (dd, 1H, CH CH2),
1
in orange powder. Yield: 0.362 g (0.56 mmol, 56%). H NMR
(400 MHz, CDCl3, Me4Si): d 1.31, 1.33 (d, 6H, CH(CH3)2 in p-
cymene), 2.30 (s, 3H, CH3 in p-cymene), 3.06 (m, 1H, CH(CH3)2
in p-cymene), 3.41 (m, 1H, NHH), 3.52 (m, 1H, CH(NH2)(C6H5)),
3.71 (m, 1H, CH(N–SO2)(C6H5)), 5.15 (d, 1H, CH CHH), 5.57–
5.73 (overlap of [1H, CH CHH] and [4H, C6H4 in p-cymene]),
6.34–7.09 (overlap of [1H, NHH] and [1H, CH CH2] and [14H,
Ar–H]). 13C NMR (100 MHz, CDCl3, Me4Si): d 18.96 (CH3–
C6H4 in p-cymene), 22.20, 22.67 (CH(CH3)2 in p-cymene), 30.63
(CH(CH3)2 in p-cymene), 69.01 (CH(NH2)(C6H5)), 71.98 (CH(N–
SO2)(C6H5)), 80.07, 80.81, 81.97, 85.09, 94.97, 104.07 (CH3–C6H4
in p-cymene), 114.66 (CH CH2), 125.02, 126.07, 126.85, 127.05,
127.14, 127.69, 128.10, 129.04, 136.28, 137.84, 138.70, 139.26 (Ar),
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2338–2347 | 2339
©

145.14 (CH CH2). Anal. Calcd. for C32H35ClN2O2RuS: C, 59.29;
H, 5.44; N, 4.32. Found: C, 58.79; H, 5.85; N, 4.05.
deposition of AIBN and the mixture was vigorously stirred during
polymerization for 24 h.
Preparation of the supported Ru-complex (C)
(3) Precipitation polymerization of methyl methacrylate and
ethyleneglycol dimethacrylate (F_PP)79
SiO2 (2 g, Aerosil 200, Degussa, surface area: 200 m2g-1) was
calcined at 773 K for 2 h, and then an absolute toluene solution
(50 mL) of p-styryltrimethoxysilane (0.31 g, 1.4 mmol) was re-
fluxed with the resulting SiO2 at 383 K for 18 h under N2. Followed
by Sohxlet extraction and washing with hexane several times, p-
styryl-functionalized SiO2 (A) was obtained after drying under
vacuum. The surface density of the p-styryl moiety was estimated
by elemental analysis (1.0 nm-2).
The Ru-complex (B) (60 mg, 0.093 mmol) was impregnated with
(A) (2 g) in anhydrous CHCl3 (40 mL) in the presence of AIBN
(5 mg), and then the mixture was refluxed at 333 K for 24 h under
N2. After Sohxlet extraction, the obtained solid was washed with
anhydrous CH2Cl2 several times and dried under vacuum. Loading
of Ru was estimated by XRF analysis (Ru 0.43 wt%).
(E) (0.45 g), methyl methacrylate (75 mg), ethyleneglycol dimethy-
lacrylate (150 mg), and AIBN (12 mg) were stirred in a mixture
of CH2Cl2 and toluene (108 mL, 2/1 v/v) in a 250 mL round
bottomed flask. The mixture was degassed in an ultrasonic bath
for 10 min, and then the flask was attached to a rotary evaporator,
submerged in a thermostatically controlled oil bath and rotated
slowly (ca. 10 r.p.m) to minimize turbulence. The temperature was
increased from 293 K to 333 K for 2 h, and the polymerization was
conducted at 333 K for 24 h. The obtained solid was filtrated and
washed with CH2Cl2 and methanol, and then dried under vacuum.
(4) Hydrolysis polymerization of Si(OCH3)4 (F_HP)71
Both TMOS (Si(OCH3)4) (1.3 g) and H2O (0.52 g) were admitted
by a chemical vapour deposition technique to (E) (0.4 g) in a
batch reactor and the hydrolysis polymerization of TMOS was
performed at 393 K. Then the whole closed system was evacuated
at 333 K for 12 h.
Coordination of the template (D) to the supported Ru-complex (C)
The sodium salt of chiral template (R)-1-(2-fluorophenyl)ethanol
(D) (17 mg, 0.1 mmol) was added to the solution of the supported
Ru-complex (C) (2.0 g, Ru 0.08 mmol) in dehydrated CH2Cl2
(30 mL). After stirring for 24 h at room temperature, the
mixture was filtered, and the obtained sample was washed with
CH2Cl2 several times and dried under vacuum. The amount of
coordinating (D) was estimated by GC analysis of unreacted (D)
in the filtrate.
Removal of the template (D) from (F)
The methanol solution of triethylbenzylammonium chloride was
added to (F) (F_PH, F_VDP, F_PP, or F_HP) and the mixture
was stirred for 24 h at 293 K. Then the solid was filtrated and
washed with methanol several times and dried under vacuum.
The amount of the released template also was estimated by GC
and its % ee was estimated by HPLC. The racemization of the
template was negligible (released template: 95% ee (R)).
Stacking of polymer-matrix overlayers on (E) to prepare (F):
Stacking of polymer-matrix overlayers was performed on (E) by
four different polymerization methods.
Characterization
(1) Photopolymerization of ThreeBond 3026E (F_PH)
1H and 13C NMR spectra were recorded on a JEOL JMN
LAMBDA 400 spectrometer at 400 MHz and 100 MHz, respec-
tively. Me4Si was used as an internal standard. Solid-state 13C
MAS NMR spectra were measured on a JEOL JNM-ECX400
spectrometer operating at 100 MHz. A sample was set in a 6 mm-
diameter zirconia rotor with kel-F caps and rotated at 5 kHz.
UV/vis spectra for homogeneous Ru-complexes were measured
on a JASCO V-550-DS spectrometer in a transmission mode.
Diffuse-reflectance (DR) UV/vis spectra for supported Ru-
complexes were also recorded on the same apparatus using an
integral sphere. Solid samples were enclosed in quartz cells under
N2 atmosphere and evacuated under vacuum without exposure to
air.
X-ray photoelectron spectroscopy (XPS) measurement was
carried out on a VG ESCALAB 220i-XL apparatus at a base
pressure of 1 ¥10-7 Pa. X-ray source, voltage, and current were
Mg-Ka, 15 kV, and 20 mA, respectively. Binding energies were
referred to C 1 s of 284.8 eV.
X-ray fluorescence (XRF) analysis of the supported Ru-
complexes was accomplished with a JEOL JSX-3400R spectrom-
eter. The loading of Ru on the solid samples was estimated using
standard curves of Ru Ka and Si Ka. ICP measurements to
The SiO2-supported Ru-complex coordinating the template (E)
(0.5 g, Ru 0.02 mmol) was stirred with a CH2Cl2 solution (4 mL) of
commercially available ThreeBond 3026E (acrylate oligomer (65–
75%), 2-hydroxyethyl methacrylate (20–30%), acrylic acid (1.4%),
and photoinitiator (1–5%)) (0.25 g) for 0.5 h. Then the solvent
CH2Cl2 was evaporated under reduced pressure, and the resulting
solid was stirred for another 1 h. UV light irradiation of the
obtained solid was performed in a quartz cell bathed in water
for 1 h with vigorous stirring using a high-pressure 400 W Hg–
Xe lamp (ORIEL Instruments) with an optical short-wavelength
cut-off filter (l > 250 nm).
(2) Vapour deposition polymerization of styrene and
divinylbenzene (F_VDP)78
The vapour deposition polymerization of styrene and divinylben-
zenewasconductedinareactorwithconnectiontoaroundbottom
flask and a sealed glass tube as reaction chambers. The reactor with
(E) (0.5 g) and AIBN (10 mg) was evacuated at room temperature
for 0.5 h and the mixture was stirred for 1 h under vacuum. Liquid
monomers (styrene 0.1 g and divinylbenzene 0.1 g) were vaporized
from the tube at 333 K. Polymerization was initiated by the thermal
2340 | Dalton Trans., 2011, 40, 2338–2347
This journal is
The Royal Society of Chemistry 2011
©

estimate Ru amounts in solutions were performed by SPS 7800
Plasma Spectrometer, SII Seiko Instrument Inc.
is essential to achieve a novel artificial enzymatic system for a
useful catalytic reaction.
Thermogravimetric analysis (TGA) was performed with a
TGA2950 instrument (TA Instrument Inc.). Samples were dried
at 323 K over night before TGA measurements and were heated
from room temperature to 1073 K with a scan rate of 10 K min-1
under air.
Molecular imprinting can be applied to prepare a cavity with a
similar shape to a template molecule in a polymer matrix and there
are several attempts to control selective catalysis using molecularly
imprinted materials, e.g., footprint catalysts,46–48 Brønsted-acid
catalyst at the interface of SiO2 and g -Al2O3,68 phosphonate-
imprinted SiO2,49 SiO2-deposited SnO2 catalysts50 etc. Metal-
complex catalysts have also been utilized to molecularly imprint
catalysts: Co-containing polymer imprinted with a shape of an al-
dolate for aldol condensation,60 molecularly imprinted Ru and Rh
complexes for transfer hydrogenation,62,63 molecularly imprinted
Ti complexes for Diels–Alder reaction,66 SiO2-supported Rh dimer
and Rh monomer for hydrogenation.31
Ru K-edge EXAFS was measured in a transmission mode at
20 K at the NW10A station of the Photon Factory Advanced
Ring at KEK-IMSS. The energy and current of electrons in
the storage ring were 6.5 GeV and 60 mA, respectively. X-rays
stemming from the storage ring were monochromatized by a
Si(311) double-crystal monochromator. Ionization chambers filled
with pure Ar and pure Kr were used to monitor incident and
transmitted X-rays, respectively. EXAFS spectra were analyzed
using Rigaku REX2000 package (ver. 2.3.3). Threshold energy
(E0) was tentatively set at the inflection point of the absorption
edge and background was subtracted by the Cook & Sayers
method.80 Curve-fitting analysis was carried out in k-space and
k3-weighted EXAFS oscillations were Fourier transformed into
R-space. Fitting parameters were coordination number (CN), in-
We have proposed molecular imprinting of an oxide-supported
metal complex, whose ligand is utilized as a template.5,69,70,75
A
metal complex is chemically attached on an oxide surface (support
surface) and a ligand used as a template is coordinated to the
supported metal complex. Then surface SiO2-matrix overlayers
were stacked on the surface of SiO2 attaching the metal complex,
surrounding the supported metal complex. The final removal of
the template produces a cavity with a similar shape to the template
ligand on the metal site.
o-Fluoroacetopheone is reported to be one of the less
enantioselective reactants for asymmetric transfer hydrogena-
tion (ATH) in water media82–84 and the improvement of its
enantioselectivity was investigated on molecularly imprinted Ru
catalysts prepared by one of its hydrogenated products (R)-1-
(o-fluorophenyl)ethanol (D) as a template. ATH is a typical
reduction of ketone derivatives and some trials to conduct
ATH in environmentally benign water media.85–94 [(p-cymene)-
Ru{NH2CH(Ph)CH(Ph)NSO2C8H7}Cl] was reported to be a
useful catalyst for ATH of ketones in water.95–97 We have de-
signed molecularly imprinted Ru-complex catalysts for ATH
of o-fluoroacetophenone in water using hydrophobic polymer
matrices as surface-matrix overlayers. The hydrophobic organic
polymer matrices behind the template were incorporated to create
a hydrophobic reaction pocket (molecularly imprinted cavity),
which is regarded as an efficient reaction site for shape-selective
ATH reactions in water.
The first step for the preparation of a molecularly imprinted Ru
catalyst was the attachment of a Ru-complex precursor (B) with
(1R,2R)-N-p-styrenesulfonyl-1,2-diphenylethylenediamine, which
is known to be highly active for transfer hydrogenation of
ketones,98–100 on p-styryl-functionalized SiO2 (A) as shown in
Scheme 1.101–103 The p-styryl moiety of (A) was reacted with that
of (B) and the Ru-complex (B) was chemically attached on the
surface of SiO2 (C). The surface density of immobilized p-styryl
moiety was regulated to be 1 nm-2, while Ru loading was regulated
to be 0.43 wt%, which corresponds to be 0.13 Ru nm-2 on the
Aerosil 200 SiO2 surface.
Then the template (R)-1-(o-fluorophenyl)ethanol (D) was co-
ordinated to the supported Ru-complex (C). The sodium salt
of (D) was smoothly reacted with the Cl and NH2 ligands of
(C). The amounts of consumed (D) were carefully measured by
GC in both homogeneous and heterogeneous cases as shown in
Supporting Information 1.† When the homogeneous Ru-complex
(B) was reacted with the equivalent amount of NaOR of (D),
the amount of free ROH (D) was negligible (0.06 equivalent to
2
teratomic distance (R), Debye–Waller factor (s ), and correction-
of-edge energy (DE0). Phase shifts and backscattering amplitudes
for Ru–C, Ru–N, and Ru–Cl were calculated by the FEFF8 code.81
ATH (asymmetric transfer hydrogenation) reaction in water
Each catalyst (Ru 4.0 ¥ 10-3 mmol) was enclosed in a Schlenk tube
and sodium dodecyl sulfonate (SDS) (1.6 ¥ 10-2 mmol), HCO2Na
(2.0 mmol), and water (2 mL) were added to the catalyst. The
reaction mixture was stirred for 1 h, and then a reactant (0.4 mmol)
was added to the solution. The reaction mixture was vigorously
stirred at 313 K for 96 h under N2 atmosphere. The reaction
mixture was extracted with diethyl ether (3 ¥ 6 mL) and the
diethyl ether solution was washed twice with brine and dehydrated
with Na2SO4. After the solution was concentrated and diluted
to constant volume, the quantitative analysis of both reactants
and products were carefully performed by GC (Shimadzu GC-
14B, Chiraldex G-TA column, 30 m ¥ 0.25 mm ¥ 0.25 mm).
Enantio excess was estimated by HPLC (JASCO HSS-2000, Daicel
Chiralcel OD-H or OB–H column).
A used catalyst was collected by a membrane filter and washed
with methanol (3 ¥ 2 mL), followed by drying under vacuum
for recycling. The mixture of filtrate and washed methanol
was concentrated under reduced pressure, and then the residual
mixture was extracted with diethyl ether (3 ¥ 6 mL). The diethyl
ether solution was washed twice with brine and dehydrated with
Na2SO4. After the concentration of the solution, conversion and
enantioselectivity were determined by GC and HPLC, respectively.
Results and discussion
Preparation of molecularly imprinted Ru-complex catalysts for
ATH in water
Natural enzymes act as highly efficient catalysts in the presence of
water and shape-selective reaction space locates on a catalytically
active metal centre in an enzyme system. The construction of both
a catalytically active metal site and shape-selective reaction space
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2338–2347 | 2341
©

Ru) in a resultant purple solution. The addition of HCl to the
purple solution released the stoichiometric amount of free ROH
(D), forming a yellow solution. 1H NMR suggested that the NH2
moiety of (B) was reacted with NaOR of (D), releasing Cl and
forming Ru–NH coordination. The addition of HCl recovered the
original Ru(Cl)(NH2) coordination. These results indicate that the
reaction of NaOR of (D) with (B) released the Cl ligand of (B) and
H of the NH2 moiety99 and formed HCl was subsequently reacted
with NaOR to produce ROH (D), which was coordinated to the
Ru-complex.
In the case of the similar reaction of heterogeneous (C) on SiO2
and NaOR of (D), the amount of free ROH (D) in a filtrate solution
was also negligible and the further addition of HCl to the solution
did not form any free ROH (D). On the other hand, 0.94 equivalent
amount of ROH (D) to Ru was detected by GC by the addition of
HCl to the solid sample reacted with NaOR of (D). The adsorption
of free ROH (D) on (C) and the direct reaction of NaOR of (D) and
Si–OH on SiO2 were also negligible (Supporting Information 1†).
Hence NaOR of (D) was reacted with the supported Ru-complex
(C) and the template ROH (D) was coordinated to the supported
Ru-complex as shown in Scheme 1 (E).
Fig. 2 UV/vis spectra for the coordination and release of the template (D)
on the homogeneous (a) and heterogeneous (b) Ru-complexes. (a) Homo-
geneous case. (B): the Ru-complex before the reaction with NaOR of (D),
(B) + Na-(D): after the reaction with NaOR of (D), and [(B)+Na-(D)] +
HCl: after the further reaction with HCl. (b) Heterogeneous case. (C) the
supported Ru-complex on SiO2, (E) after the reaction with NaOR of (D),
and (E) + HCl: after the further reaction with HCl.
13C solid-state NMR suggested that the template ROH (D)
existed on the supported Ru-complex (E). Peaks attributed to
ROH (D) were observed on (E): 21.7 ppm (CH3), 64.6 ppm (C–
OH), and 159.7 ppm (Ar–F) as shown in Fig. 1. UV/vis spectra
of both homogeneous and heterogeneous Ru-complexes showed
that releasing of the Cl ligand from the Ru-complex by the reaction
with NaOR of (D). A peak at 456 nm on (B), which is attributed to
Cl p → Ru d transition, disappeared after the reaction of NaOR
of (D) and recovered by the further reaction with HCl as shown
in Fig. 2(a). In the case of the heterogeneous Ru-complex (E),
similar disappearance and appearance of a peak at 452 nm of (E)
was observed (Fig. 2(b)). Cl 2p XPS also agreed with the result
that the Cl ligand was released from the supported Ru-complex
on (E) (Fig. 3(a)).
Fig. 3 (a) Cl 2p and (b) Ru 3p XPS spectra of (C), (E), and (G_PH) (2 nm
height).
The local coordination structures of the supported Ru-
complexes were examined by Ru K-edge EXAFS analysis (Sup-
porting Information 2†), whose structural parameters are listed in
Table 1. The supported Ru-complex (C) before the coordination
of the template (D) had Ru–N interaction, whose coordination
˚
number (CN) was estimated to be 2.0 0.4 at 2.11 0.01 A. After
the coordination of the template (D), the average bond distances
of Ru–N(diamine ligand) and Ru–O(template) were estimated to
˚
be 2.14 0.01 A (CN = 3.4 0.4).
Then surface matrix overlayers were stacked on the surface
of SiO2 attaching the Ru-complex with the template. We con-
ducted several methods to prepare the surface matrix overlay-
ers, (1) photopolymerization of ThreeBond 3026E containing
acrylate oligomer and 2-hydroxyethyl methacrylate, denoted as
method PH, (2) vapour deposition polymerization78 of styrene
and divinylbenzene, denoted as method VDP, (3) precipitation
polymerization79,104 of methylmethacrylate and ethyleneglycol
dimethacrylate, denoted as method PP, and (4) hydrolysis poly-
merization of Si(OCH3)4, denoted as method HP. The method
PH, photopolymerization is simply performed by the irradiation
of UV light to the SiO2 sample with the monomer source. The
commercial monomer source (ThreeBond 3026E) produces an or-
ganic polymer with hydroxyl groups so that the produced polymer
matrix is suggested to be more hydrophobic than SiO2 xerogel and
more hydrophilic than styrene-divinylbenzene copolymer. Vapour
deposition polymerization and precipitation polymerization have
been reported to prepare polymer electrolyte membranes,105
organosilicon polymers,106 submicrometer particles107 and drug
carriers.108
Fig. 1 Solid and liquid-state 13C NMR spectra of (B) in CDCl3, (D) in
CDCl3, (C), and (E).
2342 | Dalton Trans., 2011, 40, 2338–2347
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©

Table 1 Structural parameters of the supported Ru-complexes obtained by curve-fitting analyses of Ru K-edge EXAFS measured at 20 Ka
˚
˚
Shell
CN
R/A
DE0/eV
s/A
Supported Ru-complex (C) Rf 1.2%
Ru–C
5.5 0.9
2.0 0.4
1.5 0.2
2.22 0.01
2.11 0.01
2.44 0.01
10
6
17
0.07
0.03
0.08
Ru–N
Ru–Cl
Template-coordinated Ru-complex (E) Rf 0.4%
Ru–C
6.4 0.7
3.4 0.4
2.33 0.01
2.14 0.01
24
7
0.10
0.08
Ru–N(O)
Template-coordinated Ru-complex with surface matrix
overlayers (F_PH)b Rf 1.2%
Ru–C
6.0 0.7
3.0 0.3
2.30 0.01
2.12 0.01
20
9
0.10
0.07
Ru–N(O)
Molecularly imprinted Ru-complex (G_PH)b Rf 0.6%
Ru–C
Ru–N
Ru–Cl
5.6 0.7
1.8 0.5
1.6 0.4
2.21 0.02
2.10 0.01
2.42 0.02
8
14
13
0.07
0.04
0.12
a
-1
k range and R range were 3–14 A and 1.0–2.4 A, respectively. b The height of the polymer matrices was 2 nm.
˚
˚
We compared the polymerization methods and amounts of
SiO2 and polymer contents were estimated by TGA (Supporting
Information 3†). The XPS peak intensity analysis for the imprinted
catalysts (G) prepared by the three polymerization methods using
different organic polymers, PH, VDP, and PP, revealed that the
stacking manner of these three polymers on SiO2 were significantly
different. For 0.5 nm-height catalysts, the order of the XPS peak
intensities was (G_PH) (1.48) < (G_VDP) (1.59) ꢀ (G_PP) (4.14),
as indicated that the PH and VDP methods significantly covered
the SiO2 surface while the PP polymerization independently
proceeded on SiO2.
stacked organic polymer matrices for molecularly imprinted Ru-
complex catalysts (G) as shown in Table 2. XPS is practical to
investigate the structures of the polymer matrices of (F) and (G).
When polymerization of an organic polymer proceeds in a layer-
by-layer manner on the surface of SiO2, the intensity ratio of the
Si 2p XPS peak of SiO2 to the C 1 s XPS peak of the organic
polymer significantly decreases accompanied with an increase in
the amount of the organic polymer matrices. If polymerization
proceeds separately from the SiO2 surface, the XPS peak intensity
ratio is expected to be larger than that in the case of layer-by-layer
polymerization.
When the amounts of the polymers increased, the photopoly-
merization PH method brought about large decreases in the XPS
peak intensity ratios: 1.48 (0.5 nm), 1.27 (1 nm), 0.74 (2 nm), and
0.57 (3 nm) as shown in Table 2. On the other hand, those of
Heights of polymer matrices in Table 2 were tentatively cal-
culated by the observed densities of polymers prepared without
Table 2 XPS peak intensity ratios (Si 2p/C 1 s), the amounts of the template released from (F), and catalytic performances for the asymmetric transfer
hydrogenation of o-fluoroacetophenonea of the Ru-complex catalysts (B), (C), and (G)
Calculated polymer
matrix heightb
XPS peak intensity
ratio (Si 2p/C 1 s)d
Amount of the template
Entry
Catalyst
(polymer content)c
per Ru released from (F)e
Conv. % (TOF/h-1)f
%ee (R)
1
2
3
4
5
6
7
8
B
—
—
—
—
—
—
98 (127)
81
80
80
90
91
87
80
84
87
—
74
62
59
54
81
C
98 (47) 97g
G_PH
G_PH
G_PH
G_PH
G_VDP
G_VDP
G_VDP
G_VDP
G_PP
G_PP
G_PP
G_PP
G_HP
0.5 nm
1 nm
2 nm (26.5 wt%)
3 nm
0.5 nm
1 nm
2 nm (27.2 wt%)
3 nm
0.5 nm
1 nm
2 nm (16.7 wt%)
3 nm
1.48
1.27
0.74
0.57
1.59
0.95
0.86
0.75
4.14
2.28
1.34
0.99
>99
99
0.89
0.77
0.64
99 (5.0) 95g
81
42
35
21 (0.4)
2
12
8
5 (0.1)
3
62 (2.0)
9
10
11
12
13
14
15
2 nm
a Reaction conditions: Ru 4 ¥ 10-6 mol, Ru/SDS/o-fluoroacetophenone/HCOONa = 1/4/100/500, 2 mL water with SDS as a surfactant, 313 K, 96 h.
The quantitative analysis of both reactants and products was carefully performed by GC using calibration curves of both compounds without an internal
standard. The sum of them (mass balance) was 95–97%. b Polymer matrix heights were calculated using the observed density of each polymer. c Polymer
content was measured by TGA analysis. d Peak intensities were estimated by multiplication of amplitude and width of each peak for Si 2p and C 1 s XPS
peaks. e The amounts of the released template (D) were estimated by GC. f TOFs were initial reaction rates until 6 h. g Isolated yield. Ru 1.6 ¥ 10-5 mol,
Ru/SDS/o-fluoroacetophenone/HCOONa = 1/4/100/500, 8 mL water with SDS as a surfactant, 313 K, 96 h.
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©

(G_VDP) were 0.95 (1 nm), 0.86 (2 nm), and 0.75 (3 nm), whose
differences were smaller than those of (G_PH). These results
suggested that styrene and divinylbenzene monomers prepared by
the VDP method initially reacted with the styryl moiety supported
on SiO2 but further polymerization tended to proceed separately
from the SiO2 surface. In the case of the PP method, the XPS peak
intensity ratios were 2.28 (1 nm), 1.34 (2 nm), and 0.99 (3 nm),
which were much larger than those of (G_PH), indicating that the
PP method was not suitable for the stacking of surface polymer-
matrix overlayers.
Indeed, we observed a significant decrease in the intensity of a
Ru 3p XPS peak on the molecularly imprinted Ru-complex cata-
lyst prepared by the PH method (G_PH) with the polymer height
of 2 nm (Fig. 3(b)), indicating that space around the supported Ru-
complex was covered by the polymer matrix overlayers as shown
in Scheme 1 (F). SEM images of (G) (Supporting Information
4†) suggested that the morphology of the SiO2 particles did not
change by the polymerization.
It was found that the elimination of the template (D) was
stoichimetric on the sample (F_PH) prepared by the PH method
(2 nm polymer height). (F_PH) was smoothly released by the
template (D), whose amount was estimated to be 0.89 equivalent
to Ru by GC. On the other hand, those on (F_VDP) and (F_PP)
were 0.77 and 0.64 per Ru, respectively, as shown in Table 2. These
results indicated that a part of the supported Ru-complex was
covered by the polymer matrices on (G_VDP) and (G_PP).
The structural parameters of the local coordination of the
molecularly imprinted Ru-complexes (F_PH) and (G_PH) were
suggested by Ru K-edge EXAFS as shown in Table 1. The CNs
and bond distances of (G_PH) were similar to those of (C).
Accompanied with the stoichiometric elimination of the template
(D), the template (D) on (F_PH) was released from the supported
Ru-complex after the polymerization and a space imprinted the
shape of the template (D) was prepared behind the template on
(G_PH).
The catalytic activity was maintained on (G_PH) with the
polymer matrices of 0.5 nm, 1 nm, and 2 nm heights. (G_PH)
with the polymer matrices of 3 nm height showed decrease in
the conversion (81% conv. after 96 h) (entry 6). It is to be noted
that the enantioexcess of (R)-1-(o-fluorophenyl)ethanol increased
from 80% ee to 91% ee on (G_PH) with the polymer matrices of
2 nm height (entry 5). No improvement of % ee was observed on
(G_PH) with 3 nm-height polymers.
In the case of (G_VDP), the o-fluoroacetophenone conversion
was only 42% with 0.5 nm polymer matrices and it decreased to 2%
after 96 h when 3 nm of polymer matrices were used. Decreases in
the activity were more conspicuous on (G_PP). Enantioexcess was
also reduced to around 60% ee, indicating that the unfavourable
polymerization by the PP method caused significant decreases in
both the activity and enantioselectivity.
We prepared a molecularly imprinted Ru-complex catalyst
using hydrophilic inorganic SiO2-matrix overlayers by hydrolysis
polymerization of TMOS (G_HP). It is to be noted that the
conversion of o-fluoroacetophenone was much smaller than those
on the supported Ru-complex catalyst (C) and the molecularly
imprinted Ru-complex catalyst with the organic polymer matrices
(G_PH) as shown in Table 2, entry 15. The polarity of support
surface for Ru-TsDPEN complex was reported to be influenced
by both reactivity and enantioselectivity for ATH in water.121 The
hydrolysis polymerization of TMOS proceeded in a layer-by-layer
manner, and the surface of SiO2 was covered by hydrophilic SiO2-
matrix overlayers with plenty of Si–OH.72–74 The hydrophilicity
of the surface SiO2-matrix overlayers was suggested to be a main
reason of the decrease in the catalytic activity on the molecularly
imprinted Ru-complex catalyst (G_HP).
The polymer prepared by the PH method was more hydrophilic
than styrene-divinylbenzene copolymer and more hydrophobic
than the SiO2 matrix overlayers. The modification of the surface
polarity by the hydrophobic organic polymer matrices of (G_PH)
was advantageous for the ATH reaction in water. The molecularly
imprinted Ru-complex catalyst (G_PH) could be reusable as
shown in Fig. 4. Furthermore, the partially hydrophilic property
of the PH polymer may have a positive interaction during the
polymerization on the surface of SiO2 with Si–OH.
Catalytic performances on the molecularly imprinted Ru-complex
catalysts for ATH of ketones in water
Catalytic reactions in water media currently have been given much
attention in the viewpoint of environmentally benign chemical
synthesis.109–114 Novel catalytic systems efficiently utilizing organic
polymers have been reported for catalysis in water media,115–120
and the properties of polymer supports can affect their catalytic
performances. It is well known that Ru-TsDPEN complex and its
derivatives are highly active for ATH of ketones,98–100 and there are
some reports in water media.95–97 The catalytic performances for
ATH of o-fluoroacetophenone, whose hydrogenated product was
imprinted, were investigated on the supported and molecularly
imprinted Ru-complex catalysts in water (Table 2).
On the homogeneous Ru-precursor complex (B), conversion
and enantioexcess of o-fluoroacetophenone ATH were 98% and
81% ee (R), respectively, after 96 h (Table 2, entry 1). On
the supported Ru-complex (C) on SiO2, similar conversion
(98%) and enantioexcess (80% ee (R)) were observed without
improvement of enantioselectivity (entry 2). On the other hand,
significant differences in the activity and enantioselectivity of
o-fluoroacetophenone ATH on the molecularly imprinted Ru-
complex catalysts were observed as shown in Table 2.
Fig. 4 Recycling of the molecularly imprinted Ru-complex catalyst
(G_PH) (2 nm height) for o-fluoroacetophenone ATH in water. Reaction
conditions: Ru 4 ¥ 10-6 mol, Ru/SDS/o-fluoroacetophenone/HCOONa =
1/4/100/500, 2 mL water with SDS as a surfactant, 313 K, 96 h.
Shape selective behaviours of the molecularly imprinted Ru-
complex catalyst (G_PH) (2 nm height) were investigated using
2344 | Dalton Trans., 2011, 40, 2338–2347
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©

Table 3 Catalytic performances for ATH reactions of various reactants on the supported and molecularly imprinted Ru-complex catalystsa
(C)
(G_PH) (2 nm)
Entry
1
Reactant
TOFb/h-1
% eec
TOFb/h-1
% eec
Ratio of TOFsd
5.4
35
88 (R)
6.5
90 (R)
2
3
4
5
40
80 (R)
84 (R)
96 (R)
78 (R)
5.0
91 (R)
88 (R)
99 (R)
87 (R)
8.0
12
12
13
36
34
39
3.0
2.8
3.1
6
7
8
20
80 (R)
57 (S)
88 (R)
1.1
82 (R)
65 (S)
89 (R)
18
25
31
2.0
5.3
0.08
0.17
a Reaction conditions: Ru 4 ¥ 10-6 mol, Ru/SDS/o-fluoroacetophenone/HCOONa = 1/4/100/500, 2 mL water with SDS as a surfactant, 313 K, 96 h.
The quantitative analysis of both reactants and products was carefully performed by GC using calibration curves of both compounds without an internal
standard. b TOFs were initial reaction rates until 6 h determined by GC. c The configurations of alcohols were determined by HPLC (Chiralcel OD-H
(entries 2 and 4–7) or OB–H (entries 1, 3, and 8)). d The ratios of TOFs: TOF on the supported catalyst (C) divided by TOF on the molecularly imprinted
catalyst (G_PH).
several ketone reactants with different shapes (Table 3). Except
2-fluorobenzophenone and methyl(2-naphthyl)ketone (entries 7
and 8), other reactants (entries 1–6) exhibited fine reaction
rates (TOFs). On the molecularly imprinted Ru-complex catalyst
(G_PH), whose Ru-complex was embedded in the organic polymer
matrices, TOFs were reduced for all reactants in Table 3, but the
degrees of decreasing the reaction rates (ratios of TOFs) were
different to each other.
The ratio of TOFs of the smallest reactant in Table 3, 2-
acetylfuran (entry 1) was 5.4, while that of o-fluoroacetophenone,
whose hydrogenated product was imprinted, was 8.0 (en-
try 2). Comparison between o-fluoroacetophenone and p-
fluoroacetophenone (entries 2 and 3) suggested that the ATH
reaction of o-fluoroacetophenone was favourable on the molec-
ularly imprinted Ru-complex catalyst (G_PH) rather than
that of p-fluoroacetophenone with a different shape to the
template. A similar result was also observed between o-
fluoroacetophenone and o-chloroacetophenone (entries 2 and
5), indicating shape selective behaviours on the molecularly
imprinted catalyst (G_PH). On the other hand, there were
no significant differences in the ratios of TOFs between p-
fluoroacetophenone and p-chloroacetophenone on the molecu-
larly imprinted catalyst (G_PH). In the cases of ethylphenylketone,
o-fluorobenzophenone, methyl(2-naphthyl)ketone (entries 6, 7,
and 8), their ratios of TOFs were 18, 25, and 31, respectively, which
were much higher than that of o-fluoroacetophenone. Reactions
of ketones with different shapes to the template inhibited on the
molecularly imprinted site on the active Ru-complex. Not only
the molecularly imprinted cavity behind the template but also the
coordination structure of the Ru-complex with a chiral diamine
ligand is suggested to be important to provide the shape selective
behaviours on the molecularly imprinted Ru-complex catalyst on
SiO2.
Kinetics of the transfer hydrogenation was not investigated
on the molecularly imprinted Ru catalysts, however the reaction
mechanism of transfer hydrogenation of aromatic ketones using
similar Ru-complexes in water/HCOONa was reported.122 The
rate-determining step was suggested to be the hydrogen transfer
step. The transition state, a pseudo six-membered ring, Ru–
H . . . (R1R2)C O . . . H–N(H)–Ru, which was calculated by DFT,
was stabilized by hydrogen bonding between the O atom of C
O
and water, and hydrogen bonding between C O and the proton
of the diamine ligand was weakened. The reaction rate of transfer
hydrogenation of ketone with electron deficient C O group (ex.
o-fluorophenyl group (entry 2)) was a little faster than that with an
electron rich C O group (ex. furyl group (entry 1)), suggesting
that the transfer hydrogenation proceeded by a similar reaction
mechanism.122
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©

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We have prepared molecularly imprinted Ru-complex catalysts
on SiO2 with hydrophobic organic polymer matrices by pho-
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hydrogenation of o-fluoroacetophenone in water. The structures of
the molecularly imprinted Ru-complex catalysts were character-
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Acknowledgements
This work was financially supported by the NEDO GSC project,
the Inoue Science Research Award from Inoue Foundation for
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