Anchored [RuCl2(p-cymene)]2 in hybrid zirconium phosphate-phosphonate coated and pillared with double-stranded hydrophobic linear polystyrene as heterogeneous catalyst suitable for aqueous asymmetric transfer hydrogenation

Article abstract of DOI:10.1039/c3dt33015j

A novel type of phosphonate-containing polystyrene copolymers 1a-e bearing an N′-alkylated TsDPEN chiral ligand and double-stranded polystyrene chains were prepared for the first time using simple radical copolymerization of 1-phosphonate styrene with (R,R)-N′-4′-vinylbenzyl-N-4- vinylbenzenesulfonyl-1,2-diphenylethylene-1,2-diamine. Through the coprecipitation of their supported Ru polystyrene copolymers 2a-e and NaH ₂PO₄ with ZrOCl₂, pillared hybrid zirconium phosphate-phosphonate-anchored Ru catalysts 3a-e and 4d₁-d ₅ were obtained as heterogeneous catalysts suitable for aqueous asymmetric transfer hydrogenation. In the aqueous asymmetric transfer hydrogenation of aromatic ketones, the anchored Ru catalysts showed good catalytic activities, chemoselectivities (～100%), and enantioselectivities (73.6% ee to 95.6% ee). The Ru catalysts retained their catalytic properties even at the fifth recycle time (92.2% conv., 92.1% ee). However, corresponding supported Ru catalyst 3d′ resulted in disappointing reusability because of the loss of ruthenium in every recycle process. The conversions of aromatic ketones were closely related to the o-, m- or p-positions of the substituents on the aromatic ring caused by shape-selective matching.

Full text of DOI:10.1039/c3dt33015j

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Anchored [RuCl2(p-cymene)]2 in hybrid zirconium
phosphate–phosphonate coated and pillared with
double-stranded hydrophobic linear polystyrene as
heterogeneous catalyst suitable for aqueous
asymmetric transfer hydrogenation†
Cite this: Dalton Trans., 2013, 42, 6513
Rui Wang, Jingwei Wan, Xuebing Ma,* Xiao Xu and Liu Liu
A novel type of phosphonate-containing polystyrene copolymers 1a–e bearing an N’-alkylated TsDPEN
chiral ligand and double-stranded polystyrene chains were prepared for the first time using simple radical
copolymerization of 1-phosphonate styrene with (R,R)-N’-4’-vinylbenzyl-N-4-vinylbenzenesulfonyl-1,2-
diphenylethylene-1,2-diamine. Through the coprecipitation of their supported Ru polystyrene copoly-
mers 2a–e and NaH2PO4 with ZrOCl2, pillared hybrid zirconium phosphate–phosphonate-anchored Ru
catalysts 3a–e and 4d1–d5 were obtained as heterogeneous catalysts suitable for aqueous asymmetric
transfer hydrogenation. In the aqueous asymmetric transfer hydrogenation of aromatic ketones, the
anchored Ru catalysts showed good catalytic activities, chemoselectivities (∼100%), and enantioselectivi-
ties (73.6% ee to 95.6% ee). The Ru catalysts retained their catalytic properties even at the fifth recycle
time (92.2% conv., 92.1% ee). However, corresponding supported Ru catalyst 3d’ resulted in disappoint-
ing reusability because of the loss of ruthenium in every recycle process. The conversions of aromatic
ketones were closely related to the o-, m- or p-positions of the substituents on the aromatic ring caused
by shape-selective matching.
Received 17th December 2012,
Accepted 12th February 2013
DOI: 10.1039/c3dt33015j
www.rsc.org/dalton
prepared by two different approaches, namely, the coprecipi-
Introduction
tation of bisphosphonic acid (H2O3P-R-PO3H2, R = organic
Clearfield first prepared and elucidated the layered Zr- group) with Zr4+ ion and the direct topotactic exchange reac-
(HPO4)2·H2O (α-ZrP)1 and Zr(PO4)(H2PO4)·H2O (γ-ZrP).2 Sub- tion with a previously formed α-ZrP matrix.10
sequently, α- and γ-structured inorganic–organic functional
In the field of heterogeneous catalysis, the tailored and pil-
derivatives with different dimensionalities or solubilities,3 lared cavity, which provides spatial channels for reactants to
ranging from single, linear chains, or layered compounds to freely access the inner catalytic sites, is an important, favor-
open-framework structures, have been extensively studied able, and essential property. However, the movement of reac-
because of their high structural flexibility and applications in tants inside the pores is greatly influenced by the interaction
various fields such as adsorption,4 membranes,5 catalysis,6 between the reactant and the internal wall of the channel as a
biotechnologies,7 and photochemistry.8 Among these com- result of their different polarities. This factor was not given
pounds, pillared zirconium phosphonates with high and rigid much attention. The majority of the reactants and products in
porous structures have received increasing interest during the catalytic processes are usually medium- to non-polar. However,
past decade because of their special physical and chemical the pores constructed by inorganic frames, such as hybrid zir-
properties.9 Generally, pillared zirconium phosphonates are conium phosphate–phosphonate with O3POH moieties, are
highly polar. Therefore, modifying the polarity of internal
channels constructed by the inorganic frame is necessary to
obtain a similar affinity between the channel wall and the reac-
tant to favor the mass transfer of reactants and products inside
the pores. Thus, we designed pillared and porous materials, as
well as modified the polarity of the inner walls, to achieve
better access of reactants to catalytic sites inside inorganic
porous backbones.
College of Chemistry and Chemical Engineering, Southwest University, Chongqing,
400715, P. R. China. E-mail: zcj123@swu.edu.cn; Fax: (+86)23-68253237;
Tel: (+86)23-68253237
†Electronic supplementary information (ESI) available: 1H NMR and 13C NMR of
monomer 1; GPC spectra; the thermogravimetric curves; IR spectra; SEM
images; HRTEM images; the isotherms and distributions of pore diameter; GC
spectra. See DOI: 10.1039/c3dt33015j
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Scheme 1 The route to Ru catalyst encapsulated in double-stranded polystyrene-pillared hybrid zirconium phosphate–phosphonate.
In this paper, we incorporated the following features to synthesized samples were determined by a Hitachi model
prepare a novel type of double-stranded poly(styrene-1-phos- H-800 transmission electron microscope (TEM) and a Tecnai
phonate styrene)-pillared hybrid zirconium phosphate–phos- G2 F20 (HRTEM) operated at 200 kV. N2 adsorption–deso-
phonate suitable for aqueous hydrogenation (Scheme 1): (1) rption analysis was conducted at 77 K on an Autosorb-1
N′-alkylated TsDPEN chiral ligand with good complexation for apparatus (Quantachrome). The specific surface areas and
metal ions, such as Ru2+; (2) inorganic Zr layers coated with pore diameters were calculated by the BET and BJH models,
double-stranded hydrophobic linear polystyrene pillared by N′- respectively. C, H, N, O, and S elemental analyses were
alkylated TsDPEN to favorably obtain access of substrates to obtained from a FLASHEA1112 automatic elemental analyzer
catalytic sites; (3) covalent immobilization of chiral N′-alkyl- instrument (Italy). Gel permeation chromatography (GPC) was
ated TsDPEN to the Zr layer through anchor points (–PO3H2); performed using a 515 HPCC pump and a Waters styragel
and (4) encapsulation of supported Ru copolymer by molding HT4 DMF column (Mw 5000 to 600 000) with a 2414 refractive
hybrid zirconium phosphate–phosphonate to overcome loss of index detector from Waters. Experiments were performed at
Ru metal by leaching.
35 °C using THF as eluent and a flow rate of 0.7 mL min−1
.
Molecular weights were reported versus monodispersed poly-
saccharide standard. The enantiomeric excess (% ee) was
determined on GC with a Chiral Cyclodex-B column (30 m ×
0.25 nm × 0.25 μm, Supelco) at 100 °C for 5 min and 5 °C
min−1; and then at 140 °C for 15 min. The retention times of
Experimental
Materials and sample characterization
All chemicals were purchased and used without further purifi- (R) or (S)-1-phenylethanol were tR = 8.95 min and tS
=
cation. Fourier transform infrared spectra were recorded on a 9.26 min, respectively. X-ray photoelectron spectroscopy
Perkin-Elmer Model GX Spectrometer using the KBr pellet measurements were performed on a Physical Electronics
method with polystyrene as a standard. Thermogravimetric ESCA 5701 instrument, equipped with a multichannel detec-
analysis (TGA) was performed on an SBTQ600 thermal analy- tor and a hemispherical analyzer. Mg Kα radiation (hν =
zer (USA) with a heating rate of 20 °C min−1 from 40 °C to 1253.6 eV) was used as the excitation source and the instru-
800 °C under flowing compressed N2 (100 mL min−1). 1H, ment was operated at a constant power of 300 W (15 kV,
13C, and 31P NMR were performed on a Bruker AV-300 NMR 20 mA). The data were acquired at a take-off angle of 45° and
instrument at 300.1, 75.0, and 121.5 MHz respectively, in a pass energy of 29.35 eV. The pressure in the analysis
which all chemical shifts were reported downfield in ppm chamber was maintained at 8 × 10−7 Pa. Charge compen-
relative to the H, C, and P resonances of TMS, chloroform-d1, sation was conducted with the adventitious C 1s peak at
and H3PO4 (85%), respectively. The morphologies of the as- 284.8 eV.
6514 | Dalton Trans., 2013, 42, 6513–6522
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Synthesis of monomer 1: (R,R)-N′-4′-vinylbenzyl-N-4-
vinylbenzenesulfonyl-1,2-diphenylethylene-1,2-diamine
pressure. The other supported Ru complexes 2b–e were also
prepared in 80.0% to 83.4% yields. The supported Ru contents
in 2a–e were in the range of 3.7 wt% to 4.0 wt% according to
ICP determination.
A dried flask (250 mL) containing K2CO3 (2.1 g, 15.0 mmol)
and (R,R)-N-4-vinylbenzenesulfonyl-1,2-diphenylethylene-1,2-
diamine (3.8 g, 10.0 mmol) was flushed thrice with Ar atmos-
phere. Anhydrous acetonitrile (50 mL) was added from a
syringe and stirred at 25 °C until the reaction mixture was
completely dissolved. Then, 30 mL of 4-vinylbenzyl chloride
(1.5 g, 10.0 mmol) acetonitrile solution was added and stirred
at 60 °C for 24 h. The reaction mixture was evaporated under
reduced pressure to obtain a yellow solid, purified by flash
column chromatography eluted with petroleum ether–chloro-
form (v/v = 5 : 1) to obtain 3.7 g of white product (76% yield).
δH (300 MHz, DMSO-d6, Me4Si) 7.44–6.88 (18 H, m, Ph–H),
6.75–6.62 (2 H, m, –CHv), 5.82 (2 H, dd, 3J = 19.8, 3.0 Hz,
General preparation of anchored Ru catalysts 3a–e and 4d1–d5
2d (70.0 mg), Et3N aqueous solution (5 mL, 10.0 wt%), and
NaH2PO4·2H2O (20.0 mg, 0.13 mmol) were added to a dried
flask (25 mL). The medium pH 8 to 9 was adjusted to pH 5
using HCO2H, and the solids were dissolved. Then,
ZrOCl2·H2O aqueous solution (70.0 mg, 0.22 mmol) was added
dropwise and stirred at room temperature (rt) for 24 h. After
the pH was adjusted to 6–7 using Et3N aqueous solution, the
reddish-brown solid was filtered off, washed with distilled
water (3 × 15 mL), and ethanol (3 × 15 mL), and dried under
reduced pressure at 40 °C to yield anchored Ru catalyst 3d
(98 mg). The other catalysts (3a–e) (95 mg to 105 mg) were also
prepared in similar yields as 3d. The anchored Ru contents in
3a–e were in the range of 1.7 wt% to 2.0 wt%, as determined
using the ICP method.
The anchored Ru catalysts 4d1–d5 with 1.0 wt% to 1.2 wt%
Ru [4d1: 113 mg; 4d2: 108 mg; 4d3 (3d): 98 mg; 4d4:
95 mg; 4d5: 93 mg] were prepared under similar conditions,
changing only the amounts of NaH2PO4·2H2O used (4d1:
10.0 mg, 0.064 mmol; 4d2: 15.0 mg, 0.096 mmol; 4d3: 20.0 mg,
0.13 mmol; 4d4: 25.0 mg, 0.16 mmol; 4d5: 30.0 mg,
0.192 mmol).
3
3
vCH2), 5.29 (2 H, dd, vinyl, J = 38.9, 10.9 Hz), 4.37 (1 H, d, J
3
= 8.4 Hz, CHN), 3.78 (1 H, d, CHN, J = 8.4 Hz), 3.37 (2 H, s,
CH2). δC (75.0 MHz, DMSO-d6, TMS) 140.6, 140.5, 140.4, 140.2,
139.5, 136.7, 135.9, 135.8, 128.5, 128.2, 127.8, 127.7, 127.4,
126.9, 126.7, 126.5, 126.3 (Ph), 117.4, 114.0 (CHvCH2), 67.1,
64.1 (CHN), 50.5 (CH2). Anal. Calcd for C31H30N2O2S: C, 75.27;
H, 6.11; N, 5.66; O, 6.47; S, 6.48. Found: C, 75.30; H, 6.14; N,
5.59; O, 6.53; S, 6.41.
Synthesis of double-stranded poly(styrene-1-
phosphonatestyrene) copolymers 1a–e
A dried flask (25 mL) containing (R,R)-N′-4′-vinylbenzyl-N-4-
vinylbenzenesulfonyl-1,2-diphenylethylene-1,2-diamine Preparation of supported Ru catalyst 3d′
(173.3 mg, 0.35 mmol) was flushed thrice with Ar atmosphere.
1d (70.0 mg), Et3N aqueous solution (5 mL, 10 wt%), and
Then, 5 mL THF was added, and the resulting mixture was
stirred for 10 min. Then, 5 mL 1-phosphonate styrene
(515.0 mg, 2.8 mmol) and 3 mL benzoyl peroxide (BPO)
(20.0 mg, 0.08 mmol) THF solutions were added from a
syringe and stirred at 80 °C for 24 h. During the reaction
process, the BPO solution (2 × 20.0 mg) was added at eight-
hour intervals. The reaction mixture was evaporated under
reduced pressure to yield a pale yellow oily liquid. Upon
adding 5 mL of ethyl acetate, the white precipitate was filtered
off, washed with CH2Cl2 (3 × 5 mL) and acetone (3 × 5 mL),
and dried at 40 °C in a vacuum tank to obtain 1a (295.6 mg) in
43.0% yield.
The other copolymers (1b–e) were prepared under similar
conditions as 1a, with only the molar ratios of styrene to
1-phosphonate styrene changed to 1 : 4, 1 : 1, 4 : 1, and 8 : 1,
respectively. 1b: 339.4 mg, 68.9%; 1c: 322.6 mg, 70.6%; 1d:
371.6 mg, 80.0%; 1e: 141.4 mg, 54.6%.
NaH2PO4·2H2O (20.0 mg, 0.13 mmol) were added to a dried
flask (25 mL). The reaction mixture at pH 8–9 was adjusted to
about pH 5 using HCO2H, and the solid was found to be dis-
solved. Then, 5.0 mL ZrOCl2·H2O aqueous solution (70.0 mg,
0.22 mmol) was added dropwise and stirred at rt for 24 h. The
white solids were filtered off, washed with distilled water (3 ×
15 mL), and dried under reduced pressure at 40 °C to yield pil-
lared hybrid zirconium phosphonate–phosphonate. Then,
100.0 mg of [RuCl2(p-cymene)]2 (30.0 mg, 49.0 mmol) was
added to the obtained solid, which was then flushed thrice
with Ar atmosphere and added to 2.0 mL of anhydrous aceto-
nitrile and 0.1 mL of Et3N, then stirred at 82 °C for 6 h. The
reddish-brown supported Ru catalyst 3d′ (123.1 mg) in 94.7%
yield was filtered off, washed with CH2Cl2 (3 × 5 mL) and
acetone (3 × 5 mL), and dried at 40 °C under reduced pressure.
The supported Ru content in 3d′ with 2.5 Ru wt% was deter-
mined by ICP.
General synthesis of supported Ru complexes 2a–e
General procedure for aqueous asymmetric transfer
hydrogenation
A dried flask (25 mL) containing 1a (20 mg) and [RuCl2-
(p-cymene)]2 (6.0 mg, 9.8 mmol) was flushed thrice with Ar Anchored Ru catalyst 3d (1.7 wt%, 15.0 mg, 2.5 × 10−3 mmol),
atmosphere. Then, 2 mL anhydrous acetonitrile and 0.1 mL 0.4 mL HCO2H–Et3N v/v = 1/3, 1.5 mL water, and 0.1 mL aceto-
Et3N were added and stirred at 82 °C for 6 h. Reddish-brown phenone (102.8 mg, 0.86 mmol) were mixed in a dried flask
supported Ru complex 2a (20.8 mg) in 80.1% yield was (25 mL). The reaction mixture was well dispersed by stirring
obtained by filtering, washing with CH2Cl2 (3 × 5 mL) and for 10 min, after which it was heated to 50 °C and stirred for
acetone (3 × 5 mL), and drying at 40 °C under reduced another 10 h. The reaction residue was combined with 5 mL
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ethyl acetate and centrifuged. Then, catalyst 3d was recovered m/n = 1.2, 1.5, 1.8, 3.6, 6.8 were determined by the comparative
in quantitative yield. The conversion, chemoselectivity of aceto- quantitative 31P NMR investigation of 1a–c and 1a–c + 85%
phenone, and enantioselectivity of (R) or (S)-1-phenylethanol H3PO4 samples, according to eqn (1)–(3) (Fig. 1-b). Based on
in the upper organic phase were directly determined by GC. the quantitative principle of the 31P NMR, the peak area is
The upper organic phase was concentrated under reduced directly proportional to the number of phosphorus atoms. The
pressure, and the crude residues were purified by flash phosphorus mass in copolymers 1a–e (m2) could be calculated
column chromatography to obtain pure 1-phenylethanol.
according to eqn (1):
m1 ꢀ 85:2%
Sp
Ss
M
H3PO4
¼
ð1Þ
m2
M p
Results and discussion
Synthesis and characterization of double-stranded
poly(styrene-1-phosphonatestyrene) copolymer
where Sp is the peak area of added H3PO4, Ss denotes the total
areas of peaks, m1 denotes the added mass of H3PO4, MH PO is
the molar mass of H3PO4, and Mp is the molar mass of
phosphorus.
Double-stranded poly(styrene-1-phosphonatestyrene) copoly-
mer with functional phosphonate group (–PO3H2) and N′-alky-
lated TsDPEN chiral ligand was utilized to build a chemical
and spatial microenvironment suitable for the catalytic
process. The phosphonate (–PO3H2) played a crucial role in
anchoring poly(styrene-1-phosphonatestyrene) to the frame-
work of hybrid zirconium phosphate–phosphonate by the
coprecipitation of –PO3H2 and NaH2PO4 with ZrOCl2. N′-Alky-
lated TsDPEN had a good coordination of [Ru(p-cymene)Cl2]2
by the imino group (–NH–). The double-stranded copolymers
1a–e bearing different amounts of anchor points (–PO3H2) and
N′-alkylated TsDPEN chiral ligands were prepared by simple
radical copolymerization in THF at 80 °C for 24 h in the pres-
ence of BPO as radical initiator.11
The chemical structures and compositions of copolymers
1a–e could be elucidated by 31P NMR spectra. Aside from the
31P NMR data of poly(1-phosphonate styrene) at 12.5 (q), 5.1
(s), 1.0 (s) and 0.0 ppm (s), the new and wide 31P NMR peaks
for copolymers 1a–e emerged in the range of δ 20 to 30 ppm,
which resulted from the 31P absorption of chain segments
copolymerized by monomer 1 with 1-phosphonatestyrene
(Fig. 1-a). The chemical compositions of copolymers 1a–e with
3
4
Then, m and n values can be calculated as follows:
m2
M p
M1
m3 ꢁ
ꢀ M2
m ¼
ð2Þ
ð3Þ
m2
Mp
n ¼
where m3 is the mass of the sample, M1 is the molar mass of
monomer 1, and M2 is the molar mass of monomer 2.
The GPC analysis (Fig. 1-c) shows that the weight-average
molecular weights (Mw) of 1a–e were 974.1, 937.4, 883.7, 883.0,
and 759.5 kDa, respectively. Narrow polydispersity index (PDI)
values between 1.05 and 1.28 were also found. However, these
data might be inaccurate since monodisperse polysaccharide
standards were used for calibration while the double-stranded
copolymers 1a–e were cross-linked and brushed. The Mw and
PDI values of 1a–e decreased and increased, respectively with
increased m/n ratio from 1.2 to 6.8. Relatively narrow PDIs
(1.08 to 1.03) were also found for 1a–d at m/n ≤ 3.6. Further-
more, from IR spectra, the stretching vibrations of N–H at
3358 cm−1, Ph–H at 3060, 3029 cm−1, CvC at 1631, 1597,
1510 cm−1 and –PO3H2 at 1091, 1059 cm−1 illustrated that
N′-alkylated TsDPEN ligands were covalently attached to the
framework of double-stranded copolymers 1a–e.
Using 1d as an example, the porous structure, thermal stabi-
lity and surface morphology were further characterized by nitro-
gen adsorption–adsorption isotherm, TGA, SEM, and AFM
(Fig. 2). From the TG curves, the thermal decomposition of 1d
occurred in two steps: first, the weight loss of 3.3% between
50 °C and 200 °C corresponded to the surface-bound or inter-
calated water absorbed in the pores; second, the total weight
loss of 62.4% between 200 °C and 800 °C corresponded to the
decomposition of organic fragments. The nitrogen adsorption–
desorption isotherm, performed at 77 K, was linear to the P/P0
axis at relatively low P/P0 range (0–0.8) and convex to the P/P0
axis in the high range of P/P0 = 0.8–1.0. From the calculated
data, the pore size distribution (PSD) suggested the existence of
non-uniform micropores (<2 nm) and mesopores (2 nm to
10 nm), having several representative sharp peaks in the range
Fig. 1 The 31P NMR and GPC spectra of copolymer 1c.
6516 | Dalton Trans., 2013, 42, 6513–6522
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2d. Furthermore, as can be seen from nitrogen adsorption–
desorption isotherms (Fig. 3), some sized nanopores below
10 nm in 2d disappeared, and surface area and pore volume
decreased from 16.3 m2 g−1 and 0.8 × 10−2 cm3 g−1 to 2.0 m2
g−1 and 0.1 × 10−2 cm3 g−1, respectively, which demonstrated
that [RuCl2(p-cymene)]2 particles occupied those nanopores.
Crucially, it was very important that the remaining mesopores
in 2d at about 0.5, 1.0, 1.8, and 5.0 nm provided the diffu-
sional channels for substrates to smoothly access the catalytic
sites of entrapped [RuCl2(p-cymene)]2 in the catalytic process.
Compared with copolymer 1d, TGA of the corresponding sup-
ported Ru complex 2d showed that organic weight loss
between 150 °C and 800 °C increased from 62.6 wt% to 77.0 wt%.
Therefore, [RuCl2(p-cymene)]2 species exclusively and suc-
cessfully complexed with chiral imino (–NH) functional groups
in the framework of poly(styrene-1-phosphonatestyrene) co-
polymers 2a–e.
Fig. 2 TGA, SEM and AFM of double-stranded copolymer 1d.
Anchoring of Ru complex into hybrid zirconium
phosphate–phosphonate
Although double-stranded poly(styrene-1-phosphonatestyrene)
copolymer had good coordination with [RuCl2(p-cymene)]2,
the Ru(II) ions escaped from the porous backbone of 2a–e
owing to the flexibility of the polystyrene chain, especially in
the catalytic process. To reduce the loss of Ru(II) ions, 2a–e
were further anchored into the rigidly porous backbone of
hybrid zirconium phosphate–phosphonate through the easy
coprecipitation of mixed NaH2PO4 and 2a–e containing phos-
phonate (–PO3H2) with Zr4+ ions. Two strategies were formu-
lated to encapsulate [RuCl2(p-cymene)]2 into the porous
framework of hybrid zirconium phosphate–phosphonate. The
first approach was the coprecipitation of NaH2PO4 and 2a–e
bearing different amounts (m/n) of anchor points (–PO3H2) as
Fig. 3 The nitrogen adsorption–desorption isotherms of 1d and 2d.
of about 0.5 nm to 10 nm (Fig. 3). The average pore diameter,
pore volume, and surface area of 1d were 1.0 nm, 0.2 × 10−2
cm3 g−1 and 2.0 m2 g−1, respectively. The AFM images showed
that the particles were well dispersed in water (1 mg 1d in 5 mL
water) and had 100.8 nm mean diameter and 10.1 nm mean
height (Fig. 2). However, loosely accumulated clusters of 1d
(1 μm to 5 μm) were observed by SEM owing to being badly dis-
persed in ethanol. Therefore, SEM and AFM images could be
considered to mirror the surface morphologies of 1d in solid
state and aqueous medium, respectively.
−
precursors with ZrOCl2 at a molar ratio of Zr4+/H2PO4 = 2 to
afford anchored Ru catalysts 3a–e. The second approach was
the coprecipitation of different molar ratios of supported Ru
copolymer 2d to NaH2PO4 with ZrOCl2 to yield anchored Ru
catalysts 4d1–d5. The Ru contents in 3a–e and 4d1–d5 were
detected by ICP to be 1.7 wt% to 2.0 wt% and 1.0 wt% to
1.2 wt%, respectively. Their properties were characterized in
detail by TGA, IR, SEM, TEM, AFM, and nitrogen adsorption–
desorption isotherms.
Coordination of [RuCl2(p-cymene)]2 into poly(styrene-1-
phosphonatestyrene) copolymer
Anchoring of 2a–e into hybrid zirconium phosphate–phos-
In order to shed light on the coordination reaction of Ru2+ phonate. The TG curves (see ESI†) show that the 29.0%,
ions with N′-alkylated TsDPEN ligands, double-stranded co- 37.7%, 40.3%, 44.4%, and 48.1% weight losses for 3a–e
polymer 1d and its supported Ru complex 2d as a pair of com- between 150 °C and 800 °C corresponded to the thermal
parative examples were monitored by XPS, N2 adsorption– decomposition of appended organic fragments in the frame-
desorption analysis and TGA. From XPS spectra, the C1s, O1s, work of hybrid zirconium phosphate–phosphonate. The
N1s, P2p and S2p binding energies in 1d were 284.7, 532.1, amounts of 2a–e into 3a–e externally appeared to increase with
401.2, 133.3, and 168.2 eV, respectively. Upon the immobili- increasing m/n values from 1.2 to 6.8. In fact, m/n values in 2a–
zation of [RuCl2(p-cymene)]2 into copolymer 1d, the N1s e also exemplified the amount of anchor points (–PO3H2). The
binding energy decreased from 401.2 to 399.8 with a 1.4 eV more the m/n values were, the fewer anchor points (–PO3H2)
decrement. However, there were only 0.1 eV to 0.3 eV changes existed. However, the phosphonate –PO3H2 moieties were
for the other binding energies. On the other hand, the Ru3p difficult to thermally decompose below 800 °C. Therefore, the
(462.7 eV) and Cl2p (197.8 eV) binding energies with 1.4 At% thermal weight losses of 3a–e increased with the decreasing
and 0.6 At% contents were detected in supported Ru complex number of –PO3H2 moieties from 3a to 3e. On the other hand,
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Table 1 Porous properties of zirconium phosphonate-anchored Ru catalysts
a
3a–e and 4d1–d5
Surface areab
Average pore
diameterc [Å]
Pore volumec
Entry
[m2 g−1
]
[×10−2 cm3 g−1
]
1
2
3
4
5
6
7
8
1d
2d
3a
3b
3c
3d
3e
4d1
4d2
4d3 (3d)
4d4
4d5
16.3
2.0
10.3
10.2
10.5
10.5
10.9
11.5
10.4
10.7
10.7
11.5
10.6
10.3
0.8
0.1
0.8
1.6
1.7
2.1
1.8
2.7
2.1
2.4
2.8
1.7
15.2
30.3
31.1
37.2
34.6
34.1
35.0
37.2
52.1
33.4
9
10
11
12
Fig. 4 IR spectra of copolymer-supported Ru 2b and hybrid zirconium phos-
phonate-anchored Ru catalyst 3d.
a The samples were degassed at 100 °C for 5 h. b Based on multipoint
BET method. c Based on the desorption data using BJH method.
4d1–d5, prepared at various Zr4+/H2PO4 molar ratios, pos-
−
sessed 35.2, 40.6, 44.4, 43.5, and 44.5% total weight losses,
respectively over the 150 °C to 800 °C temperature range,
implying that similar amounts of 2d with 40.3% to 44.5%
weight losses were found to be immobilized into the frame-
work of hybrid zirconium phosphate–phosphonate at Zr4+/
NaH2PO4 (molar ratio) < 2.3.
The successful covalent attachment of double-stranded
copolymer supported Ru complexes 2a–e onto the inorganic
frame of hybrid zirconium phosphate–phosphonates 3a–e and
4d1–d5 was verified by infrared spectroscopy. The characteristic
peaks of –Ph and –CH2– moieties in 2a–e could also be
observed in the infrared spectra of 3a–e and 4d1–d5. For
example, the spectrum of 3d had two main differences com-
pared with that of 2d (Fig. 4). One variation was the stronger
absorption bands from 3700 cm−1 to 2500 cm−1 and
1700 cm−1 to 1500 cm−1 assigned to the O–H stretching
vibration of adsorbed waters in 3d. The other variation was the
stronger and wider absorption band from 1150 cm−1 to
900 cm−1 caused by the newly formed Zr–O–P stretching
Fig. 5 The nitrogen adsorption–desorption isotherms of 3a, b, d and 4d5.
vibration through the anchor points (–PO3H2) with Zr4+ ions. g−1 and to 2.8 × 10−2 cm3 g−1, as a result of the pillaring of
Based on the characteristic peaks of –Ph, –CH2–, and Zr–O–P double-stranded copolymers. No significant change in average
moieties in 3d, double-stranded copolymer-supported Ru 2d pore diameters (1.03 nm vs. 1.15 nm) was noted (Table 1). The
was covalently attached into the inorganic framework of 3d. quantity of the anchor points (–PO3H2) in 3a–e had a great
Similar infrared spectra to that of 3d were also observed for influence on the PSDs (Fig. 5). In the anchoring effect, at high
4d1–d5. However, there were different absorption intensities of content of anchor points (–PO3H2) in 3a (m/n = 1.2), a single
Zr–O–P stretching vibration. The smaller the molar ratios of micropore size at 1.8 nm was constructed owing to the
Zr4+ to NaH2PO4 in the preparation, the stronger the absorbing reinforcements of anchor points (–PO3H2). With an increase in
intensities of Zr–O–P stretching vibration were found at the m/n values from 1.2 to 6.8 for 3a–e, the micropores at
1150 cm−1 to 900 cm−1 (see ESI†).
1.8 nm gradually disappeared, and non-uniform micropores in
The chemical compositions (m/n) of 1a–e were elucidated the range of 0.5 nm to 5.0 nm emerged because of the weaken-
by 31P NMR to be 1.2, 1.5, 1.8, 3.6, and 6.8, respectively. The ing anchorage of fewer anchor points (–PO3H2). However,
higher the m/n values, the fewer phosphonates (–PO3H2) were upon altering molar ratios of Zr4+/H2PO4 using 2d as a pillar,
−
contained in 2a–e, implying that fewer anchor points (–PO3H2) 4d1–d5 possessed similar PSDs with non-uniform pores like 3d
could be utilized to anchor 2a–e into the frameworks of hybrid (Fig. 5).
zirconium phosphate–phosphonates. Surprisingly, upon
Surface morphology. Taking into account the close relation-
encapsulation of 2a–e with the anchor points (–PO3H2), the ship between the surface property of the catalyst and its cataly-
BET surface areas and pore volumes of 3a–e and 4d1–d5 were tic performance, the anchored Ru catalysts 3a, d were assigned
found to have sharply increased from 2.0 m2 g−1 and 0.1 × as examples to understand the surface morphologies in
10−2 cm3 g−1 to 15.2 m2 g−1 and to 52.1 m2 g−1, and to 0.8 cm3 different states by SEM and TEM.
6518 | Dalton Trans., 2013, 42, 6513–6522
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Table 2 The catalytic performances of anchored-Ru 3a–e and 4d1–d5 in
aqueous ATH reactiona
Entry
Cat.
S/C
Conv.b (%)
Yield (%)
(% ee)c
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
3a
3b
3c
3d
3e
4d1
4d2
4d3 (3d)
4d4
4d5
246
246
246
246
246
400
400
400
400
400
400
400
400
400
400
7.3
18.0
46.8
81.3
48.4
10.9
27.3
36.9
58.0
45.2
49.8
48.8
61.4
45.7
20.0
—
82.8
82.7
92.3
94.9
91.2
94.3
94.9
94.6
95.2
95.2
94.6
95.0
94.8
94.9
95.0
15.0
45.3
79.8
46.1
—
24.4
33.6
56.2
42.5
47.4
45.1
58.9
43.6
15.6
9
10
11
12
13
14
15
a Reaction conditions: acetophenone (0.1 mL, 0.86 mmol), HCO2H–
Et3N (0.5 mL, v/v = 1 : 3), 2 mL H2O, 50 °C, 16 h. b Monitored by GC.
c Determined by chiral GC, Chiral Cyclodex-B (30 m × 0.25 nm ×
0.25 μm, Supelco).
medium, preventing the problems of pollution that are
inherent with organic solvents.12 In this paper, considering
the hydrophilicity and good dispersal of O3POH moieties in
3a–e and 4d1–d5 in water, aqueous asymmetric transfer hydro-
genation (ATH) of aromatic ketones was used as a model reac-
tion to investigate their catalytic performances.
Fig. 6 (a) SEM; (b) TEM; (c) HRTEM of 3a and 3d.
The Ru(II) complex of N-tosyl-1,2-diphenylethane-1,2-
diamine (TsDPEN) is one of the most famous chiral ligands
and showed excellent catalytic performance in ATH reaction.13
However, N′-alkylated TsDPEN-derived Ru(II) catalysts proved
to be ineffective until 2009.14 Although the double-stranded
polystyrene copolymer supported Ru catalysts 2a–e were iso-
lated as N′-alkylated TsDPEN-derived Ru(II) complexes, they led
to moderate to poor yields and satisfactory enantioselectivities
in ATH reactions at S/C = 246 (7.3% ee to 81.3% ee, 82.8% ee
to 94.9% ee, Table 2, entries 1–5). Among them, 2d (m/n = 3.6)
possessed the best catalytic performance (81.3% ee, 94.9% ee).
Upon anchoring 2a–e in the framework of hybrid zirconium
phosphate–phosphonate, 3a–e with different amounts (m/n)
of anchor points (PO3H2) yielded decreased conversions of
acetophenone like other heterogeneous catalysts (10.9% to
58.0%, Table 2, entries 6–10). Anchored Ru catalyst 3a pos-
sessed a single pore size distribution at 1.8 nm and the lowest
surface area (15.2 m2 g−1), and the poorest conversion (10.9%)
of acetophenone was obtained. However, their enantio-
selectivities increased slightly as a result of the confinement
effect of the pores (94.3% ee to 95.2% ee). Anchored Ru
catalysts 4d1–d5 with similar pillars of double-stranded poly-
The powders of anchored Ru catalysts 3a, d in the solid
state primarily consisted of approximately 1 μm to 10 μm par-
ticles (Fig. 6a). After being well dispersed in water (5 mg in
10 mL), the TEM images showed that water medium was an
effective swelling system suitable for anchored Ru catalysts 3a,
d, and the particles obviously became smaller in size (100 nm
to 300 nm) owing to the hydrophilicity of O3POH moieties
(Fig. 6b). Due to the bonds between compolymers and the
flexibility of double-stranded polystyrene copolymers, ran-
domly wavy strips, in which Zr and P elements were detected
by EDS, were observed in the surfaces of anchored Ru catalysts
3a, d under HRTEM (Fig. 6c), illustrating the pillaring of
double-stranded polystyrene copolymers.
Porous structure. The BET surface areas, average pore dia-
meters, and pore volumes of hybrid zirconium phosphate–phos-
phonate anchored Ru catalysts 3a–e and 4d1–d5 in the solid
state are summarized in Table 1, and the representative nitro-
gen adsorption–adsorption isotherms of 3a, b and 4d1, d2 are
shown in Fig. 5.
Application in aqueous asymmetric transfer hydrogenation
From the viewpoint of green chemistry, reactions in which styrene 1d showed similar catalytic properties except for 4d5
water is used as a solvent have attracted much attention (45.1% to 58.9%, 94.6% ee to 95.0% ee, Table 2, entries 11
because water is an environmentally friendly and safe to 15).
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Table 3 The catalytic performances of anchored Ru 4d3 in different
conditionsa
Table 4 The catalytic performances of anchored-Ru 4d3 in different
conditionsa
Entry
S/C
Solvent (mL)
Conv.b (%)
Yield (%)
(% ee)c Entry
Conv.b (%)
Yield (%)
(% ee)c
1
2
3
4
5
6
7
8
400
400
400
400
400
400
400
400
400
400
100
200
800
CH3OH (2.0)
CH3CN (2.0)
THF (2.0)
CHCl3 (2.0)
DMF (2.0)
H2O (0.5)
H2O (1.0)
H2O (1.5)
H2O (2.0)
H2O (5.0)
H2O (1.5)
H2O (1.5)
H2O (1.5)
14.1
11.9
12.0
12.9
10.6
46.0
74.3
75.2
58.0
44.3
>99
—
—
—
—
95.0
93.9
94.0
95.0
94.0
95.6
94.0
94.2
95.0
94.7
94.9
93.3
94.5
1
2
3
4
5
6
7
8
m-OCH3
p-OCH3
o-Br
m-Br
p-Br
o-Cl
m-Cl
p-Cl
m-NO2
p-NO2
m-CH3
p-CH3
90.4
36.0
>99
98.5
96.6
97.7
>99
98.3
>99
94.9
98.2
72.8
88.9
34.6
98.5
97.8
95.1
95.4
97.9
96.4
97.6
92.2
96.7
70.8
91.9
86.0
94.8
90.7
90.4
89.8
95.6
90.1
73.6
74.5
95.2
56.4
—
44.4
72.3
73.8
57.8
41.7
98.0
85.9
50.7
9
9
10
11
12
13
10
11
12
86.9
52.2
a Reaction conditions: acetophenone (0.1 mL, 0.86 mmol), HCO2H–
a Reaction conditions: acetophenone (0.1 mL, 0.86 mmol), HCO2H– Et3N (0.5 mL, v/v = 1 : 3), S/C = 100, 1.5 mL H2O, 50 °C, 16 h.
Et3N (0.5 mL, v/v = 1 : 3), 50 °C, 16 h. b Monitored by GC. c Determined b Monitored by GC. c Determined by chiral GC, Chiral Cyclodex-B (30 m
by chiral GC, Chiral Cyclodex-B (30 m × 0.25 nm × 0.25 μm, Supelco).
× 0.25 nm × 0.25 μm, Supelco).
(Table 4). However, these conversions were closely related to
the o-, m- or p-positions of the substituents on the aromatic
ring. When the substituents of OCH3 and CH3 were in the
p-position of the aromatic ketones, the conversions sharply
decreased from 90.4% and 98.2% to 36.0% and 72.9%,
respectively (Table 4, entries 2, 12) caused by the mismatch
between the bulkier spatial volumes of the substrates and the
microporous spaces in 4d3. In addition, decreased enantio-
selectivities (86.0% ee and 56.4% ee) were also found. Similar
shape-selective catalyses were observed for Br, Cl, and NO2
substituents in the p-position, although they yielded excellent
conversions (94.9% to 98.3%, Table 4, entries 5, 8, 10).
The effects of solvents and substrate/catalyst molar ratios
on the catalytic performance were further investigated using
anchored Ru catalyst 4d3 (3d) as an example. Significantly
reduced conversions of acetophenone (10.6% to 14.1%) in
organic solvents were observed as expected as a result of the
poor swelling properties of 4d3 in organic solvents, such as
ethanol, and even the contraction in THF, CH3CN, and CHCl3
(Table 3, entries 1–5). To improve the catalytic activity of 4d3,
the effects of temperature, amount of water consumed, and
substrate/catalyst molar ratio (S/C) on reactivity, and enantio-
selectivity were also investigated and screened out. Table 3
entries 6–10 shows that the conversion of acetophenone was a
function of used water volume. At the lowest amount of water
used (0.5 mL), a strong inhibiting influence on the conversion
The recovery and reuse of anchored Ru catalyst
of acetophenone was found (46.0%) because of its poor mass At the end of the catalytic reaction, the anchored Ru 4d3 was
diffusion. At a higher amount of water volume used (5.0 mL), readily recovered from the reaction mixture by simple centrifu-
the poor conversion (44.3%) was caused by the diluted concen- gal separation, washed with ethyl acetate and water, and
tration of acetophenone and catalyst. The best catalytic per- reused without any other further treatment. To investigate the
formance (73.8%, 94.2% ee) was found at 1.5 mL water. Molar reused catalytic properties of the anchored Ru catalyst 4d3, the
ratios of substrate/catalyst (S/C) played a great role in catalytic supported Ru catalyst 3d′ as a reference catalyst was prepared
performance. The excellent conversion of acetophenone first by precipitation of double-stranded polystyrene copolymer
(>99%) was observed at S/C = 100 (Table 3, entry 11). Hence, 1d with Zr4+ ion to form the framework of hybrid zirconium
the optimum catalytic properties of acetophenone (>99% phosphate–phosphonate and then by complexation of
conv., 94.9% ee) catalyzed by 4d3 was obtained under the fol- [RuCl2(p-cymene)]2 (Scheme 2).
lowing conditions: 1.0 mol% Ru (S/C = 100), 50°, 0.86 mmol
Considering the different preparation sequences of
acetophenone in 1.5 mL water for 16 h. The specifically selec- anchored Ru catalyst 4d3 (3d) and supported Ru catalyst 3d′,
tive hydrogenation of CvO to C–O bond was observed without these catalysts had significant differences in conversions of
side reactions such as further hydrogenolysis of benzyl alcohol acetophenone in the recycled catalytic reactions (Fig. 7). Fig. 7
to ethylbenzene and aromatic ring to cyclohexane, which are shows that the activity of supported Ru catalyst 3d′ decreased
generally found in Pd,Rh-catalyzed hydrogenation reaction.15
sharply upon repeated recycling (only 52.3% conv. at the fifth
The optimized protocol was expanded to various aromatic recycled time). However, anchored Ru catalyst 3d retained its
ketones. Any electron-rich (OCH3 and CH3) or -poor (NO2 catalytic properties even at the fifth recycled times (92.2%
and X) aromatic ketones could yield excellent conversions conv., 92.1% ee). To determine the reason for this difference,
6520 | Dalton Trans., 2013, 42, 6513–6522
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phosphonate. Therefore, the preparation strategy through the
first complexation of [RuCl2(p-cymene)]2 and final molding of
hybrid zirconium phosphate–phosphonate was particularly
important for anchored Ru 4d3 to inhibit Ru from escaping
from the anchored pores.
Conclusions
A series of double-stranded polystyrene copolymers bearing N′-
alkylated TsDPEN chiral ligands and phosphonate (–PO3H2),
with good coordination for [RuCl2(p-cymene)]2 by imino func-
tional groups, were prepared by simple radical copolymeriza-
tion using different monomer molar ratios. Through the
anchor points of phosphonate (–PO3H2), the pillared and
anchored Ru hybrid zirconium phosphate–phosphonates
coated with hydrophobic linear polystyrene over the inner
surface of the Zr layers were obtained by the coprecipitation of
polystyrene copolymer-supported Ru complexes and NaH2PO4
with ZrOCl2. Compared with the copolymer-supported Ru cata-
lyst, zirconium phosphate–phosphonate-anchored Ru catalyst
was suitable for aqueous asymmetric transfer hydrogenation
and had good catalytic properties after recycling because of
the minimal loss of Ru. The preparation procedure known as
the “first complexation of ruthenium and then molding of
inorganic backbone” was essential to prevent Ru from escap-
ing from the anchored pores. Given the shape-selective restric-
tion of porous and pillared zirconium phosphate–
phosphonate, the substituents in the p-position of the aro-
matic ketones showed relatively poor conversions in the ATH
reaction.
Scheme 2 The synthetic route to supported Ru catalyst 3d’.
Acknowledgements
The work was supported by the National Science Foundation
of China (grant 21071116) and Chongqing Scientific Foun-
dation, P. R. China (CSTC, 2010BB4126).
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6522 | Dalton Trans., 2013, 42, 6513–6522
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