Constraining titanium tartrate in the interlayer space of layered double hydroxides induces enantioselectivity

Article abstract of DOI:10.1016/j.jcat.2010.02.006

Kagan-Medona and Sharpless titanium tartrate complexes (Ti(IV)TA_m, subscript m represents the coordination ratio of l-tartaric acid to the Ti center in the complex) have been intercalated into the interlayer of layered double hydroxides (LDHs) by anionic exchange method using Mg/Al-CO₃ LDH as the precursor. Titanium tartrate-intercalated LDHs (designated Mg/Al-Ti(IV)TA_m LDHs) with varied interlayer spacing were produced by tuning the area unit charge (A_c) of the brucite-like layer from 0.24 to 0.44 nm². The interlayer spacing decreases from 1.87 to 1.44 nm with the increase in A_c. The interlayer titanium tartrate anions are present in an interdigitated bilayer arrangement. The bidimensional interlayer space can be swollen, and thus accommodates the reactants in the interlayer. The titanium tartrate complex constrained in the LDH interlayer region shows enhanced asymmetric induction in the heterogeneous sulfoxidation of pro-chiral methyl phenyl sulfide.

Full text of DOI:10.1016/j.jcat.2010.02.006

Journal of Catalysis 271 (2010) 79–87
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Constraining titanium tartrate in the interlayer space of layered double
hydroxides induces enantioselectivity
Huimin Shi, Chenguang Yu, Jing He *
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Kagan-Medona and Sharpless titanium tartrate complexes (Ti(IV)TA , subscript m represents the coordi-
m
Received 22 December 2009
Revised 30 January 2010
Accepted 2 February 2010
Available online 5 March 2010
nation ratio of
layered double hydroxides (LDHs) by anionic exchange method using Mg/Al–CO
Titanium tartrate-intercalated LDHs (designated Mg/Al–Ti(IV)TA LDHs) with varied interlayer spacing
were produced by tuning the area unit charge (A ) of the brucite-like layer from 0.24 to 0.44 nm . The
interlayer spacing decreases from 1.87 to 1.44 nm with the increase in A . The interlayer titanium tartrate
anions are present in an interdigitated bilayer arrangement. The bidimensional interlayer space can be
swollen, and thus accommodates the reactants in the interlayer. The titanium tartrate complex con-
strained in the LDH interlayer region shows enhanced asymmetric induction in the heterogeneous sulfox-
idation of pro-chiral methyl phenyl sulfide.
L
-tartaric acid to the Ti center in the complex) have been intercalated into the interlayer of
3
LDH as the precursor.
m
2
c
c
Keywords:
Layered double hydroxides
Titanium tartrate complex
Enantioselectivity
Sulfoxidation
Confinement effect
Ó 2010 Elsevier Inc. All rights reserved.
1
. Introduction
With intense striving for environmentally benign chemical pro-
ies of montmorillonite host, in the presence of chiral tartrate ester,
gave an enantiomer excess comparable to that achieved by homo-
i
geneous Ti(OPr )
4
and dimethyl tartrate. Xiang et al. [8] reported
cesses or methodologies, much attention [1–3] has been paid on
the immobilization of homogeneous catalytic sites. The heteroge-
neous catalysts retain the active sites of homogeneous analogues
and facilitate perfect separation and subsequent recycle. Titanium
tartrate complexes [4] are well known as selective oxidation
homogeneous catalysts, and their immobilization has been re-
ported. An early attempt was made by Farrell et al. [5], who devel-
oped a polymer-supported soluble system by binding a single
tartrate ester unit to a polystyrene resin. A modest chiral induction
the preparation of organic–inorganic hybrid materials by grafting
a chiral tartaric acid derivative on the silica surface and in the mes-
opores of MCM-41. The enantioselectivities in the heterogeneous
system was as good as that in the homogeneous analogue in the
epoxidation of allyl alcohol.
Recently, new opportunities have emerged for the heterogeniza-
tion of metal complex catalyst, i.e. the enantioselectivity was possi-
ble to be enhanced by controlling the reaction process in mesoscale
or microscale using the cooperativity of solid surfaces [9] and the
confinement effects in a constricted system [10,11]. The confine-
ment effects in the pores or channels of zeolites [12] and mesopor-
ous silica materials [13,14] have been well revealed. Significant
improvement in catalytic performances (activity, stability and
enantioselectivity) has been achieved [15–20] by confining the cat-
alytic sites in the ‘rigid’ nano-sized pores of mesoporous supports.
Constraining [Co(salen)] complexes in the nanocages of SBA-16 im-
proved the conversion and ee in the hydrolytic kinetic resolution of
epoxides [15]. The enantioselectivity, for example, was boosted by
reducing the pore size of silica from 250 to 60 Å and further 38 Å in
the asymmetric hydrogenation of methyl benzoylformate using
rhodium and palladium complexes. The enhancement of enantiose-
lectivity logically reflected the increasing influence of spatial con-
straint [16]. Therefore, the enantioselectivity enhancement was
contributed to the predisposed access of reactant to catalytic center
by the spatial constraint of the walls of rigid pores [16] or the re-
stricted access of pro-chiral reactant to the active center generated
(
ca. 50–60% ee) in the epoxidation of allylic alcohols was achieved
under the same reaction conditions as in the Sharpless reaction. No
homogeneous result was reported as a control, but all the ee values
were lower than 90% of that in Sharpless system. A group of insol-
uble linear poly (tartrate ester)s were later employed as ligands to
i
Ti(OPr )
4
by Canali et al. [6]. The enantiomer excess in the epoxida-
tion of trans-hex-2-en-1-ol was found to vary in the range of 41–
9%, lower than the observed for the homogeneous analogue in
7
all cases. Owing to the swelling of gel-type polymers in the cata-
lytic reactions, inorganic supports have been developed with their
good chemical, mechanical and thermal stabilities. Choudary et al.
+
[
7] reported the first intercalation of Ti4 using tartrate ester as li-
gand in montmorillonite. In the asymmetric epoxidation of allylic
alcohols, the titanium centers intercalated in the interlayer galler-
*
Corresponding author. Fax: +86 10 6442 5385.
E-mail address: jinghe@263.net.cn (J. He).
0
021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.02.006

8
0
H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
by the pore concavity and curvature [17]. Even non-enantioselec-
tive catalysts showed significant asymmetric induction when an-
chored into ‘rigid’ confining nanospaces [21,22].
colloid mill rotating at 3000 rpm. In 2 min, the resulting slurry
was transferred to an autoclave for static crystallization at 373 K
for 8 h. The input Mg/Al ratio varied from 3/1 to 2/1 or 4/1. The ac-
tual Mg/Al ratios in final products were determined by ICP tech-
nique as 2.98, 1.98, and 4.03. The concentration of the alkali
solution was related to metal ion concentration in [NaOH] = 1.6
Different from zeolites and mesoporous materials, layered dou-
ble hydroxides (LDHs), a class of layer-structured materials with
tailorable interlayer galleries [23], provides ‘flexible’ confining
spaces. The confining space could be adjusted by changing the size
and arrangement of guest molecules [24–26]. The flexible inter-
layer spaces not only can fit small-sized moieties but also are capa-
ble of accommodating bulky catalytic sites. The bulky catalytic
sites are difficult or even impossible to enter the rigid pores with
fixed dimension. Additional attraction for the LDH supports is the
non-covalent interaction between the intercalated catalytic moie-
ties and host layers, which avoids the modification of chiral ligand
required by its covalent binding to support surface. The salen–Mn
h
i
2
+
3+
2ꢀ
3+
[Mg + Al ] and CO
= 2.0 [Al ]. The final precipitate was fil-
3
tered, washed thoroughly with deionized water, and dried at
353 K for 12 h.
Titanium tartrate-intercalated LDHs (designated Mg/Al–Ti(IV)-
TA
nium tartrate complex was first prepared by dissolving
acid to Ti alkyloxide solution in n-butanol. The input ratio of
m
LDHs) were prepared by the ion-exchanged method. The tita-
-tartaric
-tar-
L
L
taric acid to Ti is determined according to the input ratio of Kagan-
Medona or Sharpless titanium tartrate complex. m = 2 when the in-
(
III) complex, which was intercalated into Zn/Al-LDHs, displayed
put ratio of
the Kagan-Medona complex. m = 1 when the ratio of
to Ti equals the input ratio of the Sharpless complex.
L
-tartaric acid to Ti equals or exceeds the input ratio of
higher conversion, chemical selectivity and diastereoselectivity
than the analogues occluded in X and Y zeolites via the ‘‘ship-in-
bottle” approach in the stereoselective epoxidation of R-(+)-limo-
nene using molecular oxygen [27]. The OsO4 , though immobilized
just on LDH surface [28], showed higher activity and enantioselec-
tivity than the Kobayashi catalyst in the asymmetric dihydroxyla-
tion of olefins using 1,4-bis(9-O-dihydroquinidinyl)phthalazine as
chiral ligand. Iron (III)–porphyrin complexes were intercalated into
Zn/Al-LDHs and used as catalysts in the oxidation of cyclooctene,
cyclohexene, and cyclohexane with iodosylbenzene as oxidant
L-tartaric acid
i
Typically, Ti(OPr )
(0.008 mol) in a molar ratio of Ti/tartaric acid = 1/6 was mixed in
50 mL of n-butanol and then refluxed for 1 h. LDHs–CO as interca-
lated precursor was then introduced in a molar ratio of -tartaric
4
(0.0016 mol, 0.4 mL) and L-tartaric acid
2
ꢀ
3
L
acid/carbonate = 5/1. After 8 h reflux, the resulting solid was cen-
trifuged, washed with anhydrous ethanol, and dried under vacuum
at 353 K for 12 h. Two approaches were taken to adjust the area
unit charge of LDH layer. One is to change the initial chemical
composition of LDH precursor by varying the Mg/Al input (as intro-
[
29]. The catalytic activity of heterogeneous iron (III)–porphyrins
catalyst was found to depend on the chemical environments of iron
III)–porphyrins. The synergistic effects of LDH layers with the sup-
3
duced in the synthesis of LDHs–CO ), and the other is to input
(
L-tartaric acid in different excesses in the synthesis of the Ti(IV)TA
2
ported nanopalladium have also been proposed, in which the LDH
layers acted as basic ligands [30]. But the confinement effects re-
lated with the tunable bidimensional space have never been dis-
cussed in detail.
complex. The excess L-tartaric acid is used to modify the brucite-
like layer composition in the carboxyl deprotonation and intercala-
tion process, thereby fine-tuning the A value. Using Mg2.98Al–CO
-tartaric acid was varied in
c
3
LDHs as precursors, the ratio of Ti to
1/2, 1/6, 1/8, and 1/12.
L
In this work, LDHs were employed to support the titanium tar-
trate complex (designated Ti(IV)TA
coordination ratio of -tartaric acid ligand to Ti center in the tita-
nium tartrate complex) through simple ion-exchange approach.
The interlayer spacing of Ti(IV)TA intercalated LDHs was ratio-
nally tuned through tailoring the area unit charge (A ) of LDH lay-
ers, and the consequent confinement effects in the bidimensional
space were investigated. The confinement was found to predispose
stereo-selectivity and enhanced enantioselectivity was achieved.
m
, subscript m represents the
To prepare pristine Ti(IV)TA
2
complex, 10 mmol of
L-tartaric
n
L
acid and 5 mmol of Ti(OBu ) were mixed in 50 mL of n-butanol
4
under agitation. After 1 h reflux, the solvent was removed in rota-
tory evaporator at 353 K under reduced pressure. The residue was
2
c
kept under anhydrous atmosphere. The input ratio of
L-tartaric acid
n
to Ti(OBu ) was decreased to 1/1 to prepare Ti(IV)TA complex.
4
2.3. Characterization
Powder X-ray diffraction (PXRD) patterns were taken on a Shi-
madzu XRD-6000 diffractometer using Cu K radiation, with a step
2
. Experimental
a
size of 0.02° and scan speed of 5 deg/min. The ICP analysis was per-
formed on a Shimadzu ICPS-7500 inductively coupled plasma
2.1. Materials
emission spectrometer by dissolving the samples in dilute HNO
and H aqueous solution. The C and H element analysis was car-
ried out on an Elementar Co. Vario El elemental analyzer. The Fou-
rier transform infrared (FT-IR) spectra were recorded on a Bruker
3
L
-Tartaric acid (Aldrich, 99.5%), Ti(OPri)
phenyl sulfide (Acros, 99%), methyl phenyl sulfoxide (Aldrich,
7%), and H (30% aqueous solution) were used as received with-
out further purification. Mg(NO O, Al(NO ꢁ9H O, NaOH,
ꢁ6H
anhydrous Na CO , n-butanol, CH Cl , CH OH, CH CN, and N,N-di-
methyl formamide (DMF) are all of analytical purity. If necessary,
n-butanol was first desiccated with anhydrous MgSO for 24 h
and then distilled prior to use. CH Cl was first treated in 4 Å zeo-
lite overnight, and then distilled in CaH to extract the water.
4
(Aldrich, 97%), methyl
2 2
O
9
2 2
O
3
)
2
2
)
3 3
2
Vector 22 FT-IR spectrometer using standard KBr method at a res-
2
3
2
2
3
3
ꢀ1 13
olution of 4 cm
.
C CP/MAS NMR spectra were obtained with a
Bruker AV300 NMR spectrometer at a resonance frequency of
5.47 MHz. The chemical shifts are referred relative to TMS. TEM
images were taken on a JEOL 2011 microscope operated at
00 kV. The samples were prepared by dipping carbon-coated cop-
4
7
2
2
2
2
per TEM grids with dilute ethanol suspension. CD spectra were re-
corded on a JASCO J-810 spectropolarimeter at room temperature
2
.2. Synthesis
ꢀ
5
in CH
3
CN (c ꢂ 2 ꢃ 10 M) in 1.0 mm cells. During the measure-
Firstly, carbonate-intercalated LDHs (Mg/Al–CO
prepared using separate nucleation and aging steps [31]. Typically,
solution of 0.18 mol of Mg(NO and 0.06 mol of
ꢁ6H
Al(NO O dissolved in 122 mL of deionized water (Mg/
ꢁ9H
Al = 3/1) was mixed with a solution of 0.38 mol of NaOH and
.12 mol of Na CO dissolved in 122 mL of deionized water in a
3
LDHs) were
2
ment, the instrument was thoroughly purged with N .
a
3
)
2
2
O
2.4. Sulfoxidation
3
)
3
2
Typically, the catalytic sulfoxidation was performed as follows.
In a sealed 50 mL Erlenmeyer flask, methyl phenyl sulfide
0
2
3

H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
81
(
(
1
1.0 mmol), catalyst (equivalent to 0.050 mmol of Ti4+), and solvent
10 mL) were first bubbled with nitrogen gas, and then stirred for
h at room temperature and another 1 h at reaction temperature.
stacking obviously shift to lower 2h angles, clearly revealing the
successful ion-exchange of carbonate by Ti(IV)TA complex anions.
For Mg2.98Al–CO LDH, the sharp and intense diffractions ascribed
2
3
The reaction was started by the addition of 30% aqueous H as
2
O
2
to (0 0 3) and (0 0 6) planes appear at 2h = 11.5° and 22.5°. The ba-
sal spacing calculated from (0 0 3) reflection is 0.77 nm, consistent
with usually observed for Mg/Al-LDHs with carbonate as interlayer
anions. For the ion-exchanged product, the (0 0 3) and (0 0 6)
reflections shift to 2h = 4.2° and 9.3°. The basal spacing is increased
to 2.09 nm. Subtracting the layer thickness (0.48 nm) from the ba-
oxidant. The oxidant (in 10 mol% excess of methyl phenyl sulfide)
was added slowly using a microliter syringe under vigorous stir-
ring and with 10 s interruption between each drop. The reaction
mixture was sampled at intervals. The sample was added with
0
.03 g of Na
a 0.20 m microfilter. The filtrate was subject to Shimadzu HPLC
with a Daicel chiral OB-H column (254 nm) for ee determination
using n-hexane/i-proponal ( /v = 80/20) as flow phase, and Shima-
dzu 2010 GC–MS instrument with a silicone capillary column
poly(5% diphenyl-95% dimethylsiloxane,
25 m ꢃ 0.2 mm,
.33 film thickness) for conversion and selectivity
measurements.
2 3
SO to terminate the reaction, and then filtered using
l
sal spacing (2.09 nm), the interlayer spacing of Mg2.97Al–Ti(IV)TA
LDH is estimated to be 1.61 nm. For Mg2.98Al–CO LDH as interca-
2
3
v
lated precursor, the lattice parameter a (a = 2d(110)) is determined
as 0.306 nm. The value of a corresponds to the distance between
(
0
two metal cations and varies along with M(II)/M(III) ratio in the
pffiffiffi
l
m
brucite-like layer ða ¼ 2dðMꢀOÞÞ. For the Ti(IV)TA
2
intercalated
L-tartaric acid to Ti, a is esti-
product with an input ratio of 6 of
mated as 0.306 nm, corresponding to a Mg/Al ratio of 2.97. The
area/unit charge (A
LDH is calculated as 0.32 nm from A
c 2
) in the brucite-like layer of Mg2.97Al–Ti(IV)TA
3
. Results and discussion
2
2
c
= (a sin 60)/x, where x equals
to the ratio of M(III)/[M(II) + M(III)]. For Mg2.97Al–Ti(IV)TA
is 0.252.
2
LDH, x
3
.1. Tuned interlayer spacing of Mg/Al–Ti(IV)TA LDHs
2
It has been recognized that the interlayer spacing of LDHs de-
pends on the charge density (1/A ) of brucite-like layer [34]. Here-
in, A is to be tailored by modifying the chemical composition of
brucite-like layer with varied excess of -tartaric acid, thereby tun-
ing the interlayer spacing. The ratio of -tartaric acid to Ti was var-
ied from 6 to 2, 8 and 12. As shown in Fig. 1, the XRD reflections
characteristic of Mg/Al–Ti(IV)TA LDHs in each case were acquired.
The resulting cell parameter a, A , and interlayer spacing were
summarized in Table 1. When the input ratio of -tartaric acid to
Ti(IV)TA
2 3
was first intercalated into Mg2.98Al–CO LDH through
c
the ion-exchange approach with a constant input ratio of 5/1 of
L
-
c
tartaric acid to Al. The ion-exchange is carried out in a non-aque-
ous medium to avoid Ti(IV) hydrolysis and precipitation. The trace
amount of water (less than 8 wt.%) physically absorbed on LDHs is
used to facilitate the deprotonation of carboxylic groups of Ti(IV)-
L
L
2
TA
rationally chosen as the intercalation medium because it is an effi-
cient solvent for Ti(IV)TA complex and has also been found to
effectively swell the interlayer space [32].
In the ion-exchange of Mg2.98Al–CO LDH with Ti(IV)TA
tio of -tartaric acid to Ti was firstly set as six. The ratio exceeds the
2
complex. To ensure the success of intercalation, butanol-n is
c
L
2
Ti increases from 6 to 8, the metal composition of Mg/Al in the bru-
cite-like layer is decreased from 2.97 to 2.30 (as shown in Table 1).
3
2
, the ra-
2
The A
the Mg/Al–Ti(IV)TA
ever, A only slightly changes when the ratio of
Ti further rises to 12. No change in the interlayer spacing is thus
observed. It appears unnecessary to input more -tartaric acid.
When the ratio of -tartaric acid to Ti reduces to 2 (the exact stoi-
c
decreases from 0.32 to 0.27 nm . The interlayer spacing of
L
2
LDHs increases from 1.61 to 1.74 nm. How-
-tartaric acid to
stoichiometric ratio for the titanium tartrate coordination in Ka-
gan-Medona structure. In the powder XRD pattern shown in
Fig. 1, the reflections indexed [33] demonstrate a typical structure
of LDHs with a hexagonal lattice in an R3m rhombohedral symme-
try. The (0 0 l) reflections originating from the brucite-like layer
c
L
L
ꢀ
L
chiometric ratio for Kagan-Medona coordination structure), the
Mg/Al ratio in the brucite-like layer increases to 3.10, resulting in
2
an A
c
of 0.33 nm . The interlayer spacing of the Mg3.10Al–Ti(IV)TA
2
LDHs is 1.60 nm, nearly the same as for the Mg2.97Al–Ti(IV)TA
2
LDHs. It seems that, with a fixed brucite-like layer composition
Mg/Al = 2.98 for example) of intercalated precursor, the interlayer
spacing could be tuned only in a limited range by varying the ex-
cess input of -tartaric acid. Hence, the brucite-like layer composi-
tion of the intercalated precursor was varied from 2.98 to 1.98 and
.03. As shown in Table 1, when the Mg/Al ratio of intercalated pre-
(
L
4
2
cursor is decreased to 1.98, the A
an interlayer spacing of 1.87 nm is produced. When the Mg/Al ratio
of intercalated precursor is increased to 4.03, the A value rises to
.44 nm , and an interlayer spacing of 1.52 nm is obtained. So in
this work, A of Mg/Al–Ti(IV)TA LDHs is tuned by varying both
of the dosage of -tartaric acid and the composition of brucite-like
layer of Mg/Al–CO LDHs, thereby tuning the interlayer spacing.
With the increase in A , the electrostatic attraction between the
brucite-like layer and Ti(IV)TA anions gets weakened. It can be
seen clearly from Fig. 2 that the interlayer spacing decreases with
the increase in A
c
value reduces to 0.24 nm , and
c
2
0
c
2
L
3
c
2
c
.
2
3.2. Arrangement of Ti(IV)TA in the interlayer spaces
Fig. 3 illustrates TEM images of Mg2.97Al–Ti(IV)TA
2
LDHs in
bright (Fig. 3a and b) and dark (Fig. 3c and d) fields. The bright field
TEM images show the approximately disk-like slabs. The same
shapes are also revealed in the dark-field TEM images. From the
Fig. 1. XRD patterns of (a) Mg2.98Al–CO
with input ratio of -tartaric acid to Ti of (b) 2, (c) 6, (d) 8 and (e) 12. Inset: peak-
fitting of (1 1 0) and (1 1 3) made by Origin 7.0 in Gaussian mode.
3 2
LDH precursor and Mg/Al–Ti(IV)TA LDH
L

8
2
H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
Table 1
Chemical composition and structural parameters of Mg/Al–Ti(IV)TA
2
LDHs.
-Tartaric acid/[Ti] Interlayer spacing (nm) Proposed formula according to the chemical compositionc
L
a
b
(nm2)
Precursor
a
p
(nm) Intercalates
a
i
(nm)
A
c
II
III
II
III
(
M /M
)
(M /M
)
in the solution
h
i
i
i
i
i
4
.03
.98
0.307
0.306
4.37
3.10
0.307
0.306
0.306
0.306
0.305
0.305
0.44
0.33
0.32
0.27
0.26
0.24
6
2
1.52
1.60
1.61
1.74
1.75
1.87
2ꢀ
2ꢀ
2ꢀ
[
[
[
[
[
[
Mg0.81Al0.19(OH)
Mg0.76Al0.24(OH)
Mg0.75Al0.25(OH)
Mg0.70Al0.30(OH)
Mg0.69Al0.31(OH)
Mg0.66Al0.34(OH)
2
2
2
2
2
2
] ðTiðIVÞTA
2
2
2
2
2
2
Þ
Þ
Þ
ðC
4
ðC
4
ðC
4
ðC
4
ðC
4
ðC
4
H
4
H
4
H
4
H
4
H
4
H
4
O
6
O
6
O
6
O
6
O
6
O
6
Þ
Þ
Þ
Þ
Þ
Þ
ðCO
ðCO
ðCO
ðCO
3
3
3
3
3
Þ
Þ
Þ
Þ
Þ
0
:03
0:03
0:04
h
2
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
0:07
0:03
0:02
h
2.97
2.30
2.21
6
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
0:04
0:05
0:03
h
8
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
Þ
0
:02
0:11
0:02
h
12
6
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
h
Þ
Þ
ðCO
0
:03
0:10
0:02
i
1
.98
0.305
1.94
2ꢀ
:03
2ꢀ
] ðTiðIVÞTA
0
0:14
a
b
c
a
p
is denoted as the cell parameter a of the precursor.
2
is denoted as the cell parameter a of Mg/Al–Ti(IV)TA LDHs.
a
i
2ꢀ
Ti(IV)TA
(C H O ) (OC H ) ] .
2
is short for Kagan-Medona titanium tartrate complex, its molecular formula is [Ti
(C
2 4
4
H O
4
2 4
) (C H
4
O
)
6 2
(OC H
4 9
)
2
]. (Ti(IV)TA
2
)
represents
2ꢀ
[
Ti
2
4 4 6 4 4 9 2
dark-field images, a uniform distribution of the Ti(IV)TA
throughout the disk-like slabs is clearly observed.
2
complex
the two diolate oxygen atoms. Another tartrate ligand links to
the Ti center through one carbonyl oxygen and adjacent alcoholic
oxygen, leaving the other alcoholic and carbonyl group un-coordi-
nated. One alkoxy ligand is connected with titanium in trans loca-
tion to the coordinated carbonyl oxygen. Two bridging diolate
oxygen atoms binds the two titanium atoms together, producing
a six-coordinate, pseudo-octahedral structure for each titanium
1
3
Fig. 4 shows the C NMR spectra of gel-like and intercalated
Ti(IV)TA , which is expected to give information on the interaction
between Ti(IV)TA
trum of gel-like Ti(IV)TA
2
and the brucite-like layer. In the 1 C NMR spec-
(Fig. 4a), one broad signal centered at
82.4 ppm, associated with the carboxyl carbon atoms, is observed.
3
2
2
1
For pristine
L
-tartaric acid, this resonance is present at 176.5 ppm.
2
center. The intercalation of Ti(IV)TA into the LDH interlayer space
The downshift from 176.5 to 182.4 ppm indicates the coordination
of carbonyl oxygen to Ti center [35,36]. The resonance associated
with alcoholic carbon shifts to 87.1 ppm and gets broad, which is
causes better resolution and downfield shift of 182.4 and 87.1 res-
onances (Fig. 4b). The better resolution reflects the more definite
and homogeneous chemical environment of the coordinated car-
boxylate and alcoholic carbons. The signal, originating from the
carboxylic carbon coordinated with titanium, shifts from 182.4 to
184.3 ppm. The downshift is ascribed to the deprotonation of car-
boxylic groups that interact with brucite-like layer. The resonance
at 172.1 ppm attributed to the un-coordinated carboxylic carbons
is not disturbed by the intercalation. Due to the deprotonation of
the two coordinated carboxyls in one complex molecule, the Ti(IV)-
present at 74.3 ppm for
of alcoholic oxygen to Ti center. The non-disturbed signals at
71.6 ppm for carboxyl carbon and 72.5 ppm for alcoholic carbon
L-tartaric acid, revealing the coordination
1
result from the presence of un-coordinated carboxyl and alcoholic
groups. The coordination of alkoxide with Ti is confirmed by the
resonance at 65.5, 30.6, 19.0, and 13.1 ppm, which are attributed
to the butoxy carbons. The butoxy groups result from the trans-
i
esterification of Ti(O Pr) with n-butanol as solvent [37,38]. The
TA
2
should be divalent anion. The strong electrostatic attraction
anions causes
coordination of
L
-tartaric acid with Ti center in the Ti(IV)TA
2
com-
between the brucite-like layer and the Ti(IV)TA
2
plex prepared in this work thus resembles the Kagan-Medona
structure [36]. The complex bears a hexacoordinate dimeric config-
uration, as shown in Fig. 5a. In this dimeric structure, each tita-
nium atom is facially coordinated by one tartrate ligand through
the decrease in electronic density of the coordinated alcoholic car-
bon, thus giving a shift from 87.1 to 89.0 ppm.
Using Materials Studio Program, the dimensions of Ti(IV)TA
2
molecule is estimated to be 1.00 ꢃ 1.29 ꢃ 1.08 nm. The interlayer
space in each case (1.52–1.87 nm as observed by XRD patterns)
of Mg/Al–Ti(IV)TA
complex while is insufficient to the dimension of twofold Ti(IV)-
TA . An interdigitated bilayer arrangement of Ti(IV)TA in the
2 2
LDHs exceeds the dimensions of one Ti(IV)TA
2
2
interlayer gallery is proposed, with the coordinated carboxylate
groups pointed to the brucite-like layer. The alkoxy groups tend
to get adjacent through hydrophobic interaction with the alkoxy
2
groups in another Ti(IV)TA complex. From the dimensional size
~
of Ti(IV)TA
2
, its cross-section area parallel to the ( ~a ; b) plane of bru-
2
cite-like layer should be 1.39 nm (1.29 ꢃ 1.08 nm). The area per
2
charge of Ti(IV)TA
ger than A for all the brucite-like layers involved in this work. So,
to compensate the layer charge, Ti(IV)TA anions prefer to adopt a
2
in this cross-section is 0.695 nm , which is lar-
c
2
minimal projection on the brucite-like layer, giving a vertical inter-
digitated bilayer arrangement. Fig. 5b shows the schematic struc-
2
ture for Mg/Al–Ti(IV)TA LDHs.
To investigate the arrangement of the interlayer Ti(IV)TA
2
an-
ions along ( ~a ; b) plane and the interlacing degree of the adjacent
Ti(IV)TA anions, the area occupancy of Ti(IV)TA anions ( ) com-
pensating the charge of the brucite-like layer is defined.
= 0.695 ꢃ O /A . O is the charge occupancy by interlayer Ti(IV)-
TA anions. = 1, when the summed cross-section area of inter-
layer Ti(IV)TA anions exactly equals the ( ~a ; b) plane area of single
brucite-like layer. In that case, the interlayer Ti(IV)TA anions are
~
2
2
v
v
c
c
c
2
v
~
Fig. 2. Dependence of interlayer spacing on A
c
value for Mg/Al–Ti(IV)TA
2
LDHs.
2

H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
83
Fig. 3. Bright field (a and b) and dark-field (c and d) TEM images of Mg2.97Al–Ti(IV)TA
2
LDH.
densely arranged one by one.
v
< 1, when the adjacent Ti(IV)TA
2
anions are isolated in the interlayer.
Ti(IV)TA
plane. The more the adjacent Ti(IV)TA
larger
LDHs with similar interlayer spacing,
v
> 1, when the adjacent
anions interlace in the direction parallel to the ( ~a ; b)
~
2
2
anions are interlaced, the
is. It can be seen from Table 2 that, for the Mg/Al–Ti(IV)TA
v
2
v
increases along with the
ratio of interlayer Ti(IV)TA anions to co-existing tartrate. In other
word, with more co-intercalated tartrate to compensate the charge
2
of the brucite-like layer, the Ti(IV)TA anions in the interlayer
space are arranged less densely. Therefore, the arrangement of
~
interlayer Ti(IV)TA
2
anions along the ( ~a ; b) plane can be altered
through the co-intercalation of small anions while the interlayer
spacing is fixed.
3.3. Catalytic sulfoxidation
The Mg/Al–Ti(IV)TA
dation of pro-chiral methyl phenyl sulfide (MPS) using aqueous
as oxidant. The results are shown in Tables 3–5.
The catalytic oxidation of MPS using Mg2.97Al–Ti(IV)TA
catalysts was first performed in various solvents (Table 3). In DMF
Entry 1), the reaction affords methyl phenyl sulfoxide (MPSO) as
2
LDHs were evaluated by the catalytic oxi-
2 2
H O
2
LDHs as
(
the corresponding oxidation product with a selectivity of 95%,
but the conversion is quite low even in 17 h. In acetone (Entry
2
), the catalyst exhibits higher activity but no MPSO selectivity.
The solvent with higher polarity (DMF) favors the MPSO selectivity,
but disfavors the catalytic activity. Decreasing the polarity of the
Fig. 4. Solid state 13C CP NMR spectra of (a) Ti(IV)TA
solvent promotes the activity but results in over-oxidation. CH
CN
CN
complex, (b) Mg2.97Al–
3
2
Ti(IV)TA
2
LDH, and (c)
L-tartaric acid.
2 2
and a MeOH/CH Cl mixture are then applied as solvents. In CH
3

8
4
H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
2 2
Fig. 5. Schematic structure of (a) Ti(IV)TA and (b) Mg/Al–Ti(IV)TA LDHs (C , H , O , Ti ).
Table 2
anions along ( ~a ; b) plane on the excess amount of L-tartaric acid.
2
~
The dependence of the arrangement of the interlayer Ti(IV)TA
a
Interlayer spacing (nm)
Ti(IV)TA
2.44
2
anions/tartrate in the interlayer
v
Proposed formula according to the chemical composition
h
i
1.52
1.52
1.60
1.61
1.76
1.75
1.09
0.43
1.29
0.67
1.00
0.45
2ꢀ
2ꢀ
[Mg0.81Al0.19(OH)
[Mg0.81Al0.19(OH)
[Mg0.76Al0.24(OH)
[Mg0.75Al0.25(OH)
[Mg0.68Al0.32(OH)
[Mg0.69Al0.31(OH)
2
2
2
2
2
2
] ðTiðIVÞTA
2
2
2
2
2
2
Þ
Þ
Þ
Þ
Þ
Þ
ðC
ðC
ðC
ðC
ðC
ðC
4
4
4
4
4
4
H
H
H
H
H
H
4
4
4
4
4
4
O
O
O
O
O
O
6
6
6
6
6
6
Þ
Þ
Þ
Þ
Þ
Þ
0
:06
0:03
h
i
i
i
i
i
0.96
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
ðCO
ðCO
ðCO
ðCO
ðCO
3
3
3
3
3
Þ
Þ
Þ
Þ
Þ
0
:03
0:03
0:04
h
2.74
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
0
:07
0:03
0:02
h
0.74
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
0
:04
0:05
0:03
h
0.59
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
h
0
:06
0:10
0:01
0.27
2ꢀ
2ꢀ
2ꢀ
] ðTiðIVÞTA
0
:03
0:10
0:02
a
v
2
is denoted as the area occupancy of Ti(IV)TA anions compensating the charge of the brucite-like layer.
Table 3
The oxidation of MPS catalyzed by Mg2.97Al–Ti(IV)TA
2
LDHs in different solvents
.
Entrya
Solvent
Time (h)
Conv. (%)b
Product distribution (%)
2
Ti/H O
2
/substrate
O
O
O
S
S
Me
Ph
Me
Ph
1
2
3
4
5
6
7
DMF
Acetone
1/1.1/20
1/1.1/20
1/1.1/20
1/1.1/20
1/1.1/20
1/1.1/20
1/1.1/20
0/1.1/20
1/1.1/20
0/1.1/20
17
1.5
2
5
2
2
9
9
9
18
24
32
53
–
18
51
10
68
4
95
–
77
69
–
93
89
100
60
100
5
100
23
31
–
7
11
0
CH
3
CH
3
CH
2
CN
CN
Cl
2
MeOH/CH
MeOH/CH
MeOH/CH
2
Cl
2
Cl
2
Cl
2
2
2
v/v = 1/1)
v/v = 1/1)
v/v = 1/1)
c
8
9
1
CH
CH
3
CN
CN
40
0
0c
9
3
a
b
c
At 298 K.
Determined using GC–MS on a DB-5 column.
Mg2.98Al–CO LDH present in the reaction mixture, no Ti species introduced.
3

H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
85
Table 4
The oxidation of MPS catalyzed by Mg/Al–Ti(IV)TA
2
LDHs with different interlayer spacing.
Interlayer spacing (nm)b
Entrya
Catalyst
Solvent
Conv. (%)
Product distribution (%)c
ee (%)d (R)
O
S
O
O
S
Me
Ph
Me
Ph
1
2
3
4
5
6
7
Mg2.98Al–CO
3
LDH
0.28
–
MeOH/CH
CH CN
MeOH/CH
CH CN
MeOH/CH
CH CN
MeOH/CH
CH CN
MeOH/CH
CH CN
MeOH/CH
CH CN
MeOH/CH
CH CN
2
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
–
–
100
100
20
3
44
16
29
11
32
8
27
7
53
47
100
100
96
83
87
100
92
88
97
92
93
89
97
93
94
96
0
0
4
17
13
0
8
12
3
8
7
11
3
7
6
4
nd
nd
5
2
10
1
36
18
48
7
23
16
23
5
3
e
Ti(IV)TA
Ti(IV)TA
2
2
complex
e
3
absorbed Mg2.98Al–CO
3
LDH
0.28
1.52
1.61
1.75
1.87
3
Mg4.37Al–Ti(IV)TA
Mg2.97Al–Ti(IV)TA
Mg2.21Al–Ti(IV)TA
Mg1.94Al–Ti(IV)TA
Mg3.10Al–Ti(IV)TA
2
2
2
2
2
3
3
3
3
8
9
1.60
1.76
MeOH/CH
MeOH/CH
2
Cl
Cl
2
46
26
Mg2.10Al–Ti (IV)TA
2
2
2
a
b
c
The reaction under the heterogeneous condition was processed for 9 h at 273 K.
The thickness of the clay layer 0.48 nm was subtracted from the basal spacing d003
Determined using GC–MS on a DB-5 column.
Determined by HPLC system with a chiral OB-H column (Daicel) and a n-hexane/i-propanol (v/v = 80/20, or v/v = 90/10). ‘‘nd” is the abbreviation of ‘‘not determined”.
The 100% conversion was achieved less than 9 h.
.
d
e
Table 5
The oxidation of MPS catalyzed by Mg3.05Al–Ti(IV)TA LDH.
Entrya
Solvent
Time (h)
Conv. (%)
Product distribution (%)b
ee (%)c (R)
O
O
O
S
S
Me
Ph
Me
Ph
Ti (IV)TA complex
Mg3.05Al–Ti(IV)TA
Ti (IV)TA complex
Mg3.05Al–Ti(IV)TA
MeOH/CH
MeOH/CH
2
Cl
Cl
2
2
1
9
5
9
100
34
99
90
90
73
80
10
10
27
20
8
24
1
2
CH
CH
3
CN
CN
3
26
12
a
b
c
At 273 K.
Determined using GC–MS on a DB-5 column.
Determined by HPLC system with a chiral OB-H column (Daicel) and a n-hexane/i-propanol (
v
/v = 80/20).
(
Entry 3), the reaction turns out a conversion of 32% and a MPSO
selectivity of 77% in 2 h. Using the mixture ( /v = 1/1) of polar
MeOH and apolar CH Cl as solvent (Entry 6), the reaction turns
CH
2
Cl
2
and 2% ee in CH
3
CN). The heterogeneous catalytic oxidation
v
in MeOH/CH
2
Cl all exhibit improved enantioselectivity. The MPSO
2
2
2
was determined as R absolute configuration using the circular
dichroism (CD) spectroscopy, showing a positive/negative se-
quence in the sign of its Cotton effect on moving from longer to
shorter wavelengths [40]. The Cotton effect signal is strong for
out a conversion of 18% in 2 h and a MPSO selectivity of 93%. The
introduction of MeOH improves the catalytic activity (in compari-
son with Entry 5), because MeOH facilitates the compatibility to
aqueous H
reduction has occurred to the MPSO selectivity (Entry 7). With a
similar conversion, MeOH/CH Cl produces higher MPSO selectiv-
ity (Entry 7) than CH CN (Entry 4). In both CH CN and MeOH/
CH Cl solvents, Mg2.97Al–Ti(IV)TA LDHs (Entries 7 and 9) gives
much higher conversion than Mg2.98Al–CO LDH (Entries 8 and
0), demonstrating the major contribution of intercalated Ti(IV)-
TA to the catalytic activity.
The asymmetric sulfoxidation of MPS was carried out in CH
and MeOH/CH Cl /v = 1/1) at 273 K, with the input ratio of sub-
strate/Ti/H fixed at 20/1/1.1. A blank reaction, with the pres-
ence of Mg2.98Al–CO LDH, was first performed. As shown in
Table 4, negligible conversion has been accomplished in 9 h in
either MeOH/CH Cl or CH CN. The Payne reagent [39] formed by
CH CN and H contributes little to the oxidation. Ti(IV)TA com-
2
O
2
. In 9 h, the conversion rises to 51% while no marked
2
the product using Mg2.97Al Ti(IV)TA LDHs as heterogeneous cata-
lyst, while it is much weaker for the product of homogeneously
catalytic reaction. Only case improvement of enantioselectivity is
2
2
3
3
observed in CH
attribute to the solvent effects. But it can be further found in
CH CN that the enantioselectivity induced by Mg/Al–Ti(IV)TA
LDHs is similar to that induced by Ti(IV)TA absorbed Mg2.98Al-
LDH. However, in MeOH/CH Cl the enantioselectivity induced by
Mg/Al–Ti(IV)TA LDHs is greater than that induced by Ti(IV)TA
sorbed Mg2.98Al-LDH. Does the ee diversity in MeOH/CH Cl
CH
catalytic oxidation Mg/Al–Ti(IV)TA
rior surface or at the slab edges of the intercalated phase in CH
while in the interlayer regions in MeOH/CH Cl ? The conversion on
Mg/Al–Ti(IV)TA LDHs in CH CN, which is at the same level as that
catalyzed by Ti(IV)TA absorbed Mg2.98Al-LDH, seems consistent
with the above assumption. The interlayer Ti center is less accessi-
3
CN. The difference in the enantioselectivity could
2
2
2
3
3
2
1
2
2
2
2
3
CN
2
2
ab-
2
2
(v
2
2
and
2
O
2
3
CN involve the position where the catalysis takes place? The
LDHs took place on the exte-
CN,
3
2
3
2
2
3
2
2
3
2
O
2
2
2
3
plex in homogenous system gives complete conversion of MPS but
disappointing enantioselectivities in both solvents (8% ee in MeOH/
2

8
6
H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
Fig. 6. XRD patterns of (a) original and swollen Mg/Al–Ti(IV)TA
2
LDHs in (b) CH
3
CN and (c) MeOH/CH
2
Cl .
2
ble to the reactant, resulting in a low conversion. However, in
MeOH/CH Cl2, the conversions on Mg/Al–Ti(IV)TA LDHs are evi-
dently higher than on Ti(IV)TA absorbed Mg2.98Al-LDH.
To elucidate the differences between the catalysis in CH
MeOH/CH Cl , the interlayer swelling of Mg/Al–Ti(IV)TA
was carried out. Mg/Al–Ti(IV)TA LDHs was suspended in the sol-
vent dissolving MPS. After 1 h stirring, the solid was removed from
the suspension by centrifugation and directly exposed to XRD
characterization. The XRD patterns are shown in Fig. 6. The ob-
served shift of the basal reflections to small 2h degree confirms
The ion-exchange of Mg2.98Al–CO
has also been carried out with the input ratio of
3
LDH with Ti(IV)TA complex
-tartaric acid/
2
2
L
2
Ti = 1, exactly the stoichiometric ratio of Sharpless coordination
structure. The (0 0 3) and (0 0 6) reflections shift to 2h = 4.6° and
9.1°, as can be seen from Fig. 7a, giving a basal spacing of
1.92 nm and an interlayer spacing of 1.44 nm. The resulting
Mg3.05Al–Ti(IV)TA LDH has also been used as a catalyst in the sul-
3
CN and
2
LDHs
2
2
2
foxidation (Table 5). In CH
as 0.05 nm, giving a 12% ee. In MeOH/CH
in accord with a larger interlayer expansion (
3
CN, the interlayer expansion is observed
Cl , a 24% ee is achieved,
h = 0.35 nm).
2
2
D
the increase in the interlayer spacing (
LDHs. In MeOH/CH Cl , the Mg/Al–Ti(IV)TA
interlayer spacing of 1.52, 1.61, and 1.87 nm displays an
D
h) of Mg/Al–Ti(IV)TA
2
LDH with an original
2
Therefore, the heterogeneous catalysis restricted in the bidi-
mensional space induces asymmetric selectivity. The spatial con-
finement is resulted from the flexible interlayer spacing tuned by
the area unit charge of the brucite-like layer.
2
2
D
h of
0
.40, 0.23, and 0.08 nm, respectively. More obvious interlayer
swelling means that more reactants are accommodated in the
interlayer regions, allowing catalytic reactions to occur on the
interlayer active Ti center. Evident ee increment (Table 4) is thus
observed, from 10% on Ti(IV)TA
to 36%, 48% or 23% on Mg/Al–Ti(IV)TA
CH CN, the Mg/Al–Ti(IV)TA LDH with an original interlayer spac-
ing of 1.52, 1.61, and 1.87 nm displays an h of 0.17, 0.10, and
.05 nm respectively, which is smaller than that in MeOH/CH Cl
in each case. The biggest increment of ee (18%) is also observed
on the LDH with the biggest h (Entry 4). The inferior h accounts
for the less increment of the enantioselectivity observed in CH CN.
The swelling experiments indicate that h decreases with the
2
absorbed Mg2.98Al-LDH (Entry 3)
2
LDH (Entries 4–7). In
3
2
D
0
2
2
D
D
3
D
increase in original interlayer spacing. It is not difficult to under-
stand because the interaction between the brucite-like layer and
interlayer anions gets stronger with the decrease in A . For the
c
catalysis taking place in the interlayer regions, the ee value gener-
ally decreases with the increase in the original interlayer spacing,
consistent with the swelling behavior (Entries 5–7). But the ee
2
on Mg4.37Al–Ti(IV)TA LDH (Entry 4), which has the largest inter-
layer expansion, is not the highest one as expected. This can attri-
bute to the over-enlarged interlayer spacing (1.92 nm), which has
less steric constraint.
Increasing
v value under a similar interlayer spacing (Entries 5
and 8, Entries 6 and 9), the ee value shows little change, but the
conversion increased. This result implies that the distance of adja-
~
cent Ti(IV)TA
2
anions along ( ~a ; b) plane affects the contact of reac-
tants to the active Ti center, but has little influence on the
enantioselectivity.
Fig. 7. XRD patterns of (a) original and swollen Mg/Al–Ti(IV)TA LDH in (b) CH
and (c) MeOH/CH Cl
3
CN
2
2
.

H. Shi et al. / Journal of Catalysis 271 (2010) 79–87
87
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[
[
[
‘
Appendix A. Supplementary material
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[
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