On the Structure of Layered Double Hydroxides Intercalated with

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On the Structure of Layered Double Hydroxides Intercalated with Titanium Tartrate Complex for Catalytic Asymmetric Sulfoxidation Huimin Shi, Chenguang Yu, and Jing He* State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P. R. China ReceiVed: July 25, 2010; ReVised Manuscript ReceiVed: September 8, 2010

Titanium tartrate complex has been intercalated within the gallery spaces of MΠ/Al layered double hydroxides (LDHs) (MII ) Mg2+, Zn2+, Co2+) by ion exchange using CO32- LDHs as precursors. It is found that the intercalation hardly influences the coordination number of the titanium center while making tiny perturbation on the average Ti · · · O distances and octahedral symmetry. The complex anions are arranged in the interlayer gallery in an interdigitated bilayer with the coordinated carboxylates pointing to the brucite-like layer and the alkoxy group adjacent to the equivalent in another complex through hydrophobic interaction. The arrangement of interlayer titanium tartrate complex has been modulated by adjusting the charge density of unit brucitelike layer and the charge occupancy of the interlayer Ti(IV)TA2 anions. The intercalated structure is not collapsed until 573 K in the thermal treatment. The composition of the brucite-like layer as host hardly influences the bilayer arrangement of interlayer complex anions as guests; however, it has an impact on the host-guest interaction. Titanium tartrate complex intercalated LDHs were then applied as the catalysts for asymmetric oxidation of methyl phenyl sulfide. The structures of titanium tartrate intercalated LDHs were found to be stable enough to endure catalytic sulfoxidation. Introduction Tartrate titanium complexes, typically Sharpless and Kagantype reagents (titanium tetraisopropoxide (Ti(OPri)4) mixed with (+) or (-)-diethyl tartrate in different ratios), are well-known catalysts for the epoxidation of allylic alcohols1 and sulfoxidation.2,3 Several attempts have been reported on their heterogenization. Farrell et al.4 developed a polymer-supported soluble system by binding a single tartrate ester unit to a polystyrene resin. A group of insoluble linear poly(tartrate ester)s were later employed as ligands to Ti(OPri)4 by Canali and co-workers.5 Choudary et al.6 reported the first intercalation of Ti4+ in montmorillonite. The tartrate esters as chiral ligands were then introduced in the system when the intercalated Ti4+ was used as epoxidation catalysts. Xiang et al.7 reported a heterogenization approach by grafting a chiral tartaric acid derivative on the amorphous and mesoporous silica surfaces. Recently, new possibilities have emerged for heterogeneous chiral metal complex catalysts to elevate their efficiency and selectivity. The activity or enantioselectivity was possible to be enhanced by controlling the reaction process using the synergetic effects of the support surface8 or the confinement effects in a constricted system.9-19 Even nonenantioselective catalysts showed significant asymmetric induction when anchored into confining nanospaces.20,21 The immobilization of chiral ligand or catalyst to a support surface can be achieved by either covalent22 or noncovalent23 interactions. In comparison to a covalent linkage, the immobilization without covalent bonding involved exhibits merits in that the necessary modification of the chiral ligand to allow its covalent bonding to the support, which may make adverse impact on the catalytic activity and the enantioselectivity, is avoided. Layered double hydroxides (LDHs) are a class of * To whom correspondence should be addressed. E-mail: jinghe@ 263.net.cn. Tel: +86-10-64434897. Fax: +86-10-64425385.

naturally occurring or synthetic anionic clays with the interlayer anions readily exchanged and the composition of the brucitelike layers easily tuned. The anionic exchangeability allows LDHs to constrain both organic and inorganic anions in the bidimensional interlayer region.24 The interlayer spaces not only fit small-sized moieties but also are capable of accommodating bulky anions. The intercalation of functional large molecules, including salen-Mn(III) complexes,25 DNA,26 and polymers,27 has been reported. In our previous work, we reported for the first time the immobilization of titanium tartrate complex in the interlayer space of Mg/Al-LDHs by electrostatic attraction.28 The confinement effect was investigated by tailoring the interlayer gallery height. Although the titanium tartrate intercalated LDHs were found to be potential asymmetric sulfoxidation catalysts,28 their structures have never been studied in detail. This work focuses for the first time on the structure of interlayer titanium tartrate complex and the resulting intercalates, as well as the structure modulations and host-guest interactions. The structural stability of titanium tartrate intercalated LDHs is also investigated. Experimental Section Materials. L-Tartaric acid (Aldrich, 99.5%), Ti(OPri)4 (Aldrich, 97%), methyl phenyl sulfide (Acros, 99%), methyl phenyl sulfoxide (Aldrich, 97%), and H2O2 (30% aqueous solution) were used as received without further purification. Mg(NO3)2 · 6H2O, Zn(NO3)2 · 6H2O, Co(NO3)2 · 6H2O, Al(NO3)3 · 9H2O, NaOH, anhydrous Na2CO3, n-butanol, CH2Cl2, CH3OH, and CH3CN are all of analytical purity. If necessary, n-butanol was, prior to use, first desiccated with anhydrous MgSO4 for 24 h and then distilled. CH2Cl2 was first treated overnight with 4 Å zeolite and then distilled in CaH2 to extract the water. Preparation of LDHs. MII/Al-CO32- LDHs (as precursors for immobilization, MII ) Mg2+, Zn2+, and Co2+) were prepared

10.1021/jp106931g  2010 American Chemical Society Published on Web 09/28/2010

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using separate nucleation and aging steps.29 Typically, a solution of 0.18 mol of MII(NO3)2 · 6H2O and 0.06 mol of Al(NO3)3 · 9H2O dissolved in 122 mL of deionized water (MII/Al ) 3/1) was mixed with a solution of 0.38 mol of NaOH and 0.12 mol of Na2CO3 dissolved in 122 mL of deionized water in a 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 actual MII/Al ratios in final products were determined by ICP technique as 2.98, 3.00, and 3.17 of Mg/Al, Zn/Al, and Co/Al. The input Mg/Al ratio varied from 3/1 to 2/1 or 4/1, and the actual Mg/Al ratios in final products were determined by ICP technique as 2.98, 1.98, and 4.03. The concentration of the alkali solution was related to metal ion concentration in [NaOH] ) 1.6[MII + Al3+] and [CO32-] ) 2.0[Al3+]. The final precipitate was filtered, washed thoroughly with deionized water, and dried at 353 K for 12 h. Intercalation of Titanium Tartrate. Titanium tartrate intercalated LDHs were prepared by the ion-exchanged method using MII/Al-CO3 LDHs as precursors. The titanium tartrate complex was first prepared by dissolving L-tartaric acid to Ti alkoxide solution in n-butanol. Typically, Ti(OPri)4 (0.0013 mol, 0.4 mL) and L-tartaric acid (0.008 mol) in a molar ratio of Ti/ L-tartaric acid ) 1/6 was mixed in 50 mL of n-butanol and then refluxed for 1 h. MII/Al-CO32- LDH as intercalated precursor was then introduced in a molar ratio of L-tartaric acid/ carbonate ) 5/1. After 8 h reflux, the resulting solid was centrifuged, washed with anhydrous ethanol, and dried under vacuum at 353 K for 12 h. Using Mg2.98Al-CO3 LDHs as precursors, the ratio of Ti to L-tartaric acid was varied in 1/2, 1/6, 1/8, and 1/12. Using Mg1.98Al-CO3 LDHs and Mg4.03AlCO3 LDHs as precursors, the ratio of Ti to L-tartaric acid was varied in 1/2 and 1/6. The intercalates prepared in a tartrate/Ti molar ratio above 2 are denoted MII/ Al-Ti(IV)TA2, and the ones in a tartrate/Ti molar ratio of 1 are denoted MII/Al-Ti(IV)TA. Preparation of Solid Titanium Tartrate Complex. Typically, 10 mmol of L-tartaric acid and 5 mmol of Ti(OBun)4 were mixed in 50 mL of n-butanol under agitation. After 1 h reflux, the solvent was removed in a rotatory evaporator at 353 K under reduced pressure. The residue was kept under anhydrous atmosphere. The complexes prepared in the tartrate/Ti molar ratio of 2 and 1 are denoted Ti(IV)TA2 and Ti(IV)TA. Characterization. Powder X-ray diffraction (PXRD) patterns were taken on a Shimadzu XRD-6000 diffractometer using Cu KR radiation with a step size of 0.02° and scan speed of 5°/ min. In situ XRD measurement was performed in N2 atmosphere. The samples as unoriented powders were scanned in steps using a count time of 4 s per step. In situ TG-MS was carried out on a PE-Diamond TG-MS instrument by heating the sample from 273 to 873 K in N2 atmosphere at a rate of 5 K/min and a N2 flow rate of 10 mL/min. The ICP analysis for MII, Al, and Ti was performed on a Shimadzu ICPS-7500 inductively coupled plasma emission spectrometer by dissolving the samples in dilute HNO3 and H2O2 aqueous solution. The C and H element analysis was carried out on an Elementar Co. Vario El elemental analyzer. The Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vector 22 FT-IR spectrometer using standard KBr method at a resolution of 4 cm-1. 13C CP/MAS NMR spectra were obtained with a Bruker AV300 NMR spectrometer at a resonance frequency of 75.47 MHz. The chemical shifts are referred relative to TMS. The Ti K-edge absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were collected in the fluorescent mode at the NSRL facility (National Syn-

Shi et al. chrotron Radiation Laboratory, Hefei, China) on the beam station U7C. The storage ring was operated at 800 MeV with a stored electron current in the range 150-200 mA. A Si [111] channel-cut monochromator was employed. The scanning step is 0.5 and 1 eV in XANES and EXAFS regions. XANES spectra were normalized to an edge jump of unity. The raw data analysis was performed using the NSRL-XAFS 3.0 software package30 and IFEFFIT 1.2.11 software according to the standard data analysis procedures that took into account multiple scattering (MS) theory. Pre-edges were normalized in absorbance by fitting the spectral region from 4860 to 4960 eV (the region below the pre-edge) using a New Victoreen function and subtracting this as background absorption. The pre-edges were then normalized for atomic absorption, based on the average absorption coefficient of the spectral region from 5030 to 5600 eV (after the main-edge crest). CD spectra were recorded on a JASCO J-810 spectropolarimeter at room temperature in CH3CN (c ≈ 2 × 10-5 M) in 1.0 mm cells. During the measurement, the instrument was thoroughly purged with N2. Sulfoxidation. Typically, the catalytic sulfoxidation was performed as follows. In a sealed 50 mL Erlenmeyer flask, methyl phenyl sulfide (1.0 mmol), catalyst (equivalent to 0.050 mmol of Ti4+), and solvent (CH2Cl2/methanol, V/V ) 1/1, 10 mL) were first bubbled with nitrogen gas and then stirred for 1 h at room temperature and another 1 h at reaction temperature. The reaction was started by the addition of 30% aqueous H2O2 as oxidant. The oxidant (in 10 mol % excess of methyl phenyl sulfide) was added slowly using a microliter syringe under vigorous stirring and with 10 s interruption between each drop. The reaction mixture was sampled at intervals. The sample was added with 0.03 g of Na2SO3 to terminate the reaction and then filtered using a 0.20 µm microfilter. The filtrate was subject to Shimadzu HPLC with a Daicel chiral OB-H column (254 nm) for ee determination using n-hexane/isopropyl alcohol (V/V ) 80/20) as flow phase and a Shimadzu 2010 GC-MS instrument with a silicone capillary column (DB-5, poly(5% diphenyl-95% dimethylsiloxane, 25 m × 0.2 mm, 0.33 µm film thickness) for conversion and selectivity measurements. Results and Discussion Titanium Tartrate Structure. In our study, the pristine Ti(IV)TA2 complex solid is first investigated by FTIR, NMR, and XAS spectra to assist in understanding the structure of the interlayer tartrate titanium complex. Figure 1 shows the FTIR spectrum of Ti(IV)TA2 complex. For comparison, the spectrum for L-tartaric acid is also given. In the spectrum of Ti(IV)TA2 complex, the band at 1741 cm-1 associated with the asymmetric vibrations of carboxyl groups in L-tartaric acid shifts to 1747 cm-1. The band assigned to the symmetric vibration of carboxyl groups shifts from 1448 to 1434 cm-1. The bands assigned to asymmetric CdO-Ti vibrations in Kagan-type structure31 emerge at 1660, 1641, and 1581 cm-1. The band at 1400 cm-1 is assigned to the symmetric vibration of C)O-Ti vibrations. The band at 991 cm-1 assigned to the C-O-H bending in L-tartaric acid is not observed. The band at 1074 cm-1 originating from C-O-Ti31-33 appears. The band at 1465 cm-1 is due to the C-H bending vibration of CH2, indicative of the preservation of alkoxyl groups linked with the Ti center. The bands at 2962, 2933, and 2873 cm-1 originate from the stretching vibration of C-H in the alkoxyl groups. The coordination mode of L-tartaric acid to the Ti center is supported by the solid-state 13C CP/MAS NMR spectra shown in Figure 2. As described before,28 the appearance of coordinating diolate O-C-H and coordinating COOH signals (Figure 2b) proved

Titanium Tartrate Complex Intercalation

Figure 1. FTIR spectra of (a) L-tartaric acid, (b) Ti(IV)TA2 complex, (c) Mg2.97Al-Ti(IV)TA2, (d) Zn3.00Al-Ti(IV)TA2, and (e) Co2.23AlTi(IV)TA2.

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Figure 3. XANES spectra taken with fluorescence detection at the Ti K-edge for Ti(IV)TA2 complex (solid line) and Mg2.97Al-Ti(IV)TA2 (dashed line).

Figure 2. 13C CP/MAS NMR spectra of (a) L-tartaric acid, (b) Ti(IV)TA2 complex, (c) Mg2.97Al-Ti(IV)TA2, and (d) Zn3.00AlTi(IV)TA2.

the η2 configuration in the complex, in accordance with Kagantype structure.32 The Kagan-type complex includes a fivemembered chelate ring in [Ti(η2-tar)(η2-Htar)(OBun)]2 configuration. The detailed coordination of L-tartaric acid to titanium (coordinated number and Ti-O distances) in the Ti(IV)TA2 complex is further investigated by XAS spectroscopy. In the Ti K-edge XANES spectrum (Figure 3, solid line), triplet preedge peaks (P1, P2, and P3), which is characteristic of octahedral symmetry,34 are observed in the range of 24-12 eV before the main crest. P1 at 4967.4 eV is due to the quadrupolar transitions to t2g levels of the TiO6 octahedron. P2 at 4969.6 eV originates mostly from 1s to 3d dipolar transition to the t2g orbital of the neighboring octahedron. P3 at 4972.9 eV is associated with 1s to 3d dipolar transition to the eg orbital of the neighboring octahedron. In the postedge region, three crests (denoted A, B, and C) corresponding to the 3s f np dipole-allowed transitions are resolved. In light of previous reports,35,36 postedge peaks B and C originated from a photoelectron multiple scattering, and their coexistence supported the octahedral structure of Ti(IV)TA2 complex. Accordingly the complex structure is schematically illustrated in Figure 4a, which bears a hexacoordinate dimeric configuration. In this dimeric structure, each titanium atom is

Figure 4. Schematic structure of (a) Ti(IV)TA2 and (b) MII/ Al-Ti(IV)TA2 LDHs (C cyan b, H small gray b, O red b, Ti dark blue b, MII light blue b, Al large gray b).

facially coordinated by one tartrate ligand through 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 uncoordinated. One alkoxy ligand is connected with titanium in trans location to the coordinated carbonyl oxygen. Two bridging diolate oxygen atoms bind the two titanium atoms together, producing a sixfold-coordinate, pseudo-octahedral structure for each titanium center. The Ti · · · O distance has been obtained by fitting the EXAFS results. Figure 5A gives the experimental and Fourier transform of EXAFS spectra k[χ(k)] (k space) of Ti(IV)TA2. Fourier transform of this signal, multiplied by k3, yields the radial distribution function shown in Figure 5B. An intense peak is observed at short distance (1.32 Å) due to the backscatter of the first neighbor’s oxygen atoms. The FEFF program (FEFF 6.0) applying a curved-wave multiple-scattering theory has been used to fit the experimental EXAFS results. The results are summarized in Table 1. The σ2 is a parameter

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Figure 5. Fourier transform k3-weighed Ti-edge EXAFS spectra (A) and magnitudes spectra of EXAFS (B) of the Ti(IV)TA2 complex (a) and Mg2.97Al-Ti(IV)TA2 (b) obtained at the Ti-edge: (solid line) experimental and (dashed line) fitting. Hanning window in the range k ) 3.5-8.5 Å-1 was used.

TABLE 1: EXAFS Fit Results for Ti(IV)TA2 and Mg2.97Al-Ti(IV)TA2 (Single Shell Fitting)a sample Ti(IV)TA2 Mg2.97Al-Ti(IV)TA2

shell

N

R (Å)

σ2 (Å2)

O O O O

3.44 1.92 3.40 1.94

1.92 2.06 1.92 2.08

0.004 0.001 0.004 0.003

a Ti(IV)TA2 is short for Kagan-Medona titanium tartrate complex; its molecular formula is [Ti2(C4H4O4)2(C4H4O6)2(OC4H9)2].

associated with the disordering of a complex. As shown in Table 1, the coordination number of titanium is fitted as above 5.3, supporting the sixfold coordination observed by the XANES spectrum. Two Ti · · · O distances are resolved at 1.92 and 2.06 Å. It was reported in a Ti6(µ2-O)2(µ3-O)2(µ2-OC4H9)2(OC4H9)6(OCOCH3)8 octahedron that the Ti-O bond lengths varied from 1.742 to 2.148 Å,37 consistent with our observation. Ti-O bond length was found to increase in the order of terminal alkoxide < oxo (bridging diolate) < Ti-OdC,38 with Ti-O bonds of the terminal butoxy groups observed around 1.7537,39 and 1.81 Å,39 and the Ti-O bonds of the bridging butoxy group at 1.962 Å.37 The Ti · · · OdC bond located in the trans position of the very short bond of the terminal butoxy groups was the longest one.37 So in our case the 1.92 Å is supposed to be the average value of the terminal alkoxide and alcoholic Ti-O bonds, matching the fitted coordination number of 3.44. The 2.06 Å is assigned to the average value of the diolate and coordinated carboxyl Ti · · · O bonds, matching the fitted coordination number of 1.92. The dimensions of the Ti(IV)TA2 molecule are thus estimated as 1.00 × 1.29 × 1.08 nm from the modulation with the Materials Studio Program as shown in Figure 4a. Interlayer Ti(IV)TA2 Structure and Arrangement. The Ti(IV)TA2 complex was introduced into LDH interlayer region by the ion-exchange approach first using Mg2.98Al-CO32- LDH as the precursor. As shown in Figure 6, in each case the XRD reflections are observed typical of layered structure, indexed as a hexagonal lattice in R3jm rhombohedral symmetry. The (003) reflection shifts, from 11.5° for Mg2.98Al-CO32- (Figure 6a) to 4.2° for Mg2.97Al-Ti(IV)TA2 (Figure 6b), suggesting the successful intercalation of Ti(IV)TA2 anions. In the Ti K-edge

Figure 6. XRD patterns of (a) Mg2.98Al-CO32-, (b) Mg2.97AlTi(IV)TA2, (c) Mg3.05Al-Ti(IV)TA, (d) Zn3.00Al-CO32-, (e) Zn3.00Al-Ti(IV)TA2, (f) Co3.17Al-CO32-, and (g) Co2.23Al-Ti(IV)TA2.

XANES spectrum (Figure 3, dashed line), doublet peaks (P1 and P2), instead of a triple feature, are observed for interlayer Ti(IV)TA2. It was reported40 that P2 predominance with respect to the other two peaks was characteristic of the pre-edge region in a distorted octahedron where a relaxation of inversion symmetry occurred, and in many cases only the central peak P2 was observed.36,41,42 So the absence of characteristic P3 in our case is supposed to be due to the distorted octahedral symmetry of the interlayer Ti(IV)TA2, which is supported by the increase in σ2 from 0.001 Å2 for pristine Ti(IV)TA2 to 0.003 Å2 for interlayer Ti(IV)TA2 (Table 1). The better resolution of postedge peaks B and C indicates a quasi-stationary state of the interlayer Ti(IV)TA2. The intercalation hardly influences the coordination number of titanium center while making a tiny perturbation on the average Ti · · · O distances. As resolved in Figure 5B, the backscatter of the first neighbor’s oxygen atoms shifts to 1.43 Å for interlayer Ti(IV)TA2. The bond lengths of the interlayer Ti(IV)TA2 slightly increase to 1.92 and 2.08 Å as shown in Table 1. But the coordination number of titanium is still fitted as above 5.3. In the FT-IR spectrum (Figure 1c), the band originating from C-O-Ti remains at 1076 cm-1. But the bands assigned to the CdO-Ti vibrations are enveloped with 1630 cm-1. In the CP/MAS NMR spectrum (Figure 2c), the chemical shift characteristic of diolate OCH is observed at 89.0 ppm, the coordinating COO at 184.3 ppm, and the alkoxide group linked with titanium at 64.7, 30.8, 19.1, and 13.6 ppm. The DRS absorption maximum for Mg2.97Al-Ti(IV)TA2 is observed at 265 nm. It was reported that the absorption at 260-270 nm resulted from site-isolated Ti atoms in penta- or octahedral coordination40,43 and the absorptions above 300 nm were associated with TiO2.44 The absorption of Mg2.97Al-Ti(IV)TA2 excludes the presence of polymeric -Ti-O linkages. From the 13C CP/MAS NMR spectra shown in Figure 2, it can be seen the intercalation results in better resolution of C-O-Ti and CdO-Ti resonances, suggesting more definite and homogeneous chemical environment of the coordinated carboxylate and alcoholic carbons. The resonance at 172.1 ppm attributed to the uncoordinated carboxylic carbons is not



TABLE 2: Structural Parameters and Chemical Composition of Mg/Al-Ti(IV)TA2 LDHs precursor (MII/MIII)

apa (nm)

4.03

0.307

2.98

0.306

1.98

0.305

intercalates (MII/MIII) 4.33 4.37 3.10 2.97 2.30 2.21 2.11 1.94

aib (nm)

L-tartaric acid/[Ti] in the solution

Dcc (e/nm2)

interlayer spacing (nm)

ncd

Oc (%)

Oc′ (%)

Ot (%)

0.307 0.307 0.306 0.306 0.306 0.305 0.305 0.305

2 6 2 6 8 12 2 6

2.30 2.28 3.01 3.11 3.74 3.85 3.99 4.22

1.52 1.52 1.60 1.61 1.74 1.75 1.76 1.87

0.06 0.03 0.07 0.04 0.03 0.02 0.06 0.03

69 27 55 31 40 17 36 18

63 63 47 46 39 37 36 35

28 28 38 42 54 63 61 82

a ap is denoted as the cell parameter a of the precursor Mg/Al-CO32- LDHs. b ai is denoted as the cell parameter a of Mg/Al-Ti(IV)TA2 LDHs. c Dc is denoted as the charge density of the LDH brucite-like layer. Dc ) e · x/a2 sin 60° (x ) M(III)/[M(II) + M(III)]). d nc is denoted as the population of interlayer Ti(IV)TA2 anions in the calculated molecule formula.

disturbed by the intercalation. The carboxylic carbon coordinated with titanium shifts from 182.4 to 184.3 ppm, in accordance with the deprotonation of coordinated carboxylic groups for the electrostatic interaction with the brucite-like layers. The restriction in the interlayer space results in the shift of the coordinated alcoholic carbon from 87.1 to 89.0 ppm and the uncoordinated alcoholic carbon from 72.5 to 75.0 ppm. Due to the deprotonation of the two coordinated carboxyls in one complex molecule, the interlayer Ti(IV)TA2 should be a divalent anion. The preservation of the FT-IR band at 1740 cm-1 for COOH groups supports the observation in NMR spectrum that the uncoordinated carboxylic carbon has not been involved in the interaction with brucite-like layers. Based on the above results, an interdigitated bilayer arrangement of Ti(IV)TA2 in the interlayer gallery can be deduced, as shown in Figure 4b. The coordinated carboxylic groups are deprotonated and pointing to the brucite-like layer,; meanwhile, the alkoxy groups at the trans position tend to get adjacent through hydrophobic interaction with the alkoxy groups in adjacent Ti(IV)TA2 complexes. The interlayer spacing calculated from the XRD patterns shown in Figure 6 confirms the interdigitated bilayer arrangement of interlayer Ti(IV)TA2 anions. The basal spacing (d003) for Mg2.98Al-CO3 as the precursor has been calculated as 0.77 nm, while the basal spacing for Mg2.97Al-Ti(IV)TA2 is estimated to be 2.09 nm. By subtracting the thickness of the brucite-like layer (0.48 nm) from the basal spacing, the interlayer region height for Mg2.97Al-Ti(IV)TA2 is estimated as 1.61 nm, exceeding the size in any dimension of one Ti(IV)TA2 complex while less than the sum of Ti(IV)TA2 size in any two dimensions. The elemental analysis of Mg2.97Al-Ti(IV)TA2 (Table 2) shows the cointercalation of tartrate and carbonate anions, which facilitates the full charge balance. The intercalation of Ti(IV)TA produces an interlayer spacing of 1.44 nm (Figure 6c), corresponding to a bilayer interdigitated arrangement of Ti(IV)TA in the interlayer region. In the preparation, the arrangement of interlayer titanium tartrate complex has been modulated by adjusting the charge density (Dc) of unit brucite-like layer (Dc ) e · x/(a2 sin 60°), wherein x ) M(III)/(M(II)+M(III)) and a ) 2 × d110) and the charge occupancy (Oc) of the interlayer Ti(IV)TA2 anions. To adjust the Dc value, the Mg/Al of the brucite-like layer of the precursors and the excess amount of L-tartaric acid to Ti have both been changed. As shown in Table 2, when the Mg/Al of the brucite-like layer of the precursors is 2.98 with an initial input ratio of L-tartaric acid/Ti as 6, the Dc values of intercalates are similar to that of the precursor. Reducing the initial input ratio of L-tartaric acid/Ti to 2 results in a slight decrease in Dc (from 3.11 to 3.01 e/nm2). Increasing the initial L-tartaric acid/ Ti input ratio from 6 to 8 and further to 12 causes the Dc to

increase from 3.11 to 3.74 and further to 3.85 e/nm2, in accordance with the reduction of Mg/Al ratio. The Dc tuning only by the input ratio of L-tartaric acid/Ti at a fixed brucitelike layer composition appears limited. Thus the Mg/Al ratio of the precursor has been varied from 2.98 to 1.98 and 4.03. The Dc value of the resulting intercalates is hence modulated in the range of 2.28-4.22 e/nm2 as shown in Table 2. With increasing Dc, the arrangement density of the interlayer anions increases along the (a b, b b) plane to compensate the positive charge of the brucite-like layer. The strong Coulombic repulsion between adjacent Ti(IV)TA2 anions pushes them apart along the direction perpendicular to the (a b, b b) plane of the brucitelike layer. In addition, the Coulombic repulsions between brucite-like layers increase with increasing Dc value. Either of the above repulsions reduces the interlacement of interlayer b, b b) plane of Ti(IV)TA2 in the direction perpendicular to the (a the brucite-like layer, which results in an increase in the interlayer spacing as shown in Figure 7A. To adjust Oc, the population of interlayer Ti(IV)TA2 anions (nc) has been tuned by changing the molar ratio of tartaric acid to Ti. Oc is defined 2nc/Dc × 100%, wherein 2 is the valence state of Ti(IV)TA2 anions. In light of the dimensional size of b, b b) plane Ti(IV)TA2, its cross-sectional area parallel to the (a 2 of the brucite-like should be 1.39 nm (1.29 × 1.08 nm). The charge density of Ti(IV)TA2 in this cross section is 1.44 e/nm2, which is smaller than all the Dc values of the brucite-like layers involved in this work. When the interlayer Ti(IV)TA2 anions are arranged in a dense monolayer mode, the theoretical Oc (denoted as Oc′) can be calculated from Oc′ ) 1.44/Dc × 100%. As shown in Table 2, for the brucite-like layer with Mg/Al ) 2.98, when the ratio of L-tartaric acid to Ti is tuned as 8, nc ) 0.03, giving an Oc close to Oc′. Increasing the L-tartaric acid/ Ti ratio to reduce nc, an Oc smaller than Oc′ is observed, meaning b, b b) plane. the Ti(IV)TA2 anions are discrete along the (a Decreasing the L-tartaric acid/Ti ratio until nc increases to 0.07, an Oc larger than Oc′ is observed, meaning the interlayer Ti(IV)TA2 anions are densely arranged with adjacent Ti(IV)TA2 overlapped in b a or b b direction along the (a b, b b) plane. Similar are the observations for the brucite-like layer with Mg/Al ) 1.98 or 4.03. The cointercalation of small tartrate anions to compensate the positive charge of the brucite-like layer has been resolved by elemental analysis, as shown in Table 2. The charge occupancy of interlayer tartrate is denoted Ot. The interlayer spacing hardly changes with Ot in the case that Dc values are similar, as shown in Figure 7B. The cointercalated tartrate anions exist in the interlayer space pillared by the larger Ti(IV)TA2 anions. With similar Ot, the location and orientation of interlayer tartrate are supposed to depend on the arrangement of interlayer

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Figure 7. Dependence of interlayer spacing on Dc value for Mg/ Al-Ti(IV)TA2 (A) and relationship between charge occupancy of interlayer tartrate and interlayer spacing (B).

Ti(IV)TA2 anions. The possible arrangements of interlayer anions are schematically illustrated in Figure 8. When the interlayer Ti(IV)TA2 anions are arranged discretely (Oc < Oc′), the unoccupied interlayer space allowed the coexisting tartrate anions to take orientation diversely, either parallel or perpendicular to the (a b, b b) plane. When the interlayer Ti(IV)TA2 anions are densely arranged (Oc g Oc′), the interlayer space available for the coexisting tartrate anions was limited, accommodating a restricted orientation. Host-Guest Interactions. The intercalation then has been performed on Zn/Al-CO32- and Co/Al-CO32- LDHs (Figure 6). For Zn/Al LDHs, not all Zn3.00Al-CO32- has been transformed to Zn/Al-Ti(IV)TA2, the (00l) reflections typical of Zn3.00Al-CO32- being present. In the case of Co/Al LDHs, the absence of diffraction maxima typical of Co3.17Al-CO32suggests the disappearance of the precursor phase. The basal spacing (d003) of Zn3.00Al-Ti(IV)TA2 is 2.24 nm, giving an interlayer spacing of 1.76 nm. For Co2.23Al-Ti(IV)TA2, the basal spacing (d003) is 2.26 nm, giving an interlayer spacing of 1.78 nm. Either of the interlayer spacings is larger than that for Mg2.97Al-Ti(IV)TA2 but still less than twofold Ti(IV)TA2 size in any dimension. So the interlayer Ti(IV)TA2 anions are also arranged in interdigitated bilayer in Zn3.00Al-Ti(IV)TA2 and Co2.23Al-Ti(IV)TA2, analogous to those in Mg2.97Al-

Shi et al. Ti(IV)TA2. The chemical composition indicates the cointercalation of tartrate and carbonate anions in either Zn3.00AlTi(IV)TA2 or Co2.23Al-Ti(IV)TA2. The host-guest interaction has been of interest regarding the chemical composition of brucite-like layers. According to the reported pKa values of the metal hydroxides (11.5 for Mg(OH)2,45 10.2 for Co(OH)2,46 and 8.8 for Zn(OH)247), the hydrogen bonding between brucite-like layers and interlayer Ti(IV)TA2 (MII-O-H...O-CdO-Ti) is supposed to increase in the order of Mg2.97Al-Ti(IV)TA2 < Co2.23Al-Ti(IV)TA2 < Zn3.00Al-Ti(IV)TA2. The interaction of Ti(IV)TA2 with brucitelike layers causes the symmetric vibration of coordinated carboxylate in interlayer Ti(IV)TA2 to shift from 1400 to 1384 cm-1 for Mg2.97Al-Ti(IV)TA2 and Co2.23Al-Ti(IV)TA2, while from 1400 to 1365 cm-1 for Zn3.00Al-Ti(IV)TA2 (Figure 1). More shift in IR frequency means stronger interaction between the Zn/Al brucite-like layers and interlayer Ti(IV)TA2, which is in accordance with the pKa of hydroxides. In a previous report, the difference in the absorption frequency (∆υ) between the asymmetric and symmetric stretching vibrations of COO- group (∆υ ) υas - υs) was used to study the symmetry of the interaction between the carboxylate group and the brucite-like layer.48 For sodium tartrate, in which the two oxygen atoms of the carboxylate bear identical C-O vibrations, ∆υ was resolved as 204 cm-1. A ∆υ larger than 204 cm-1 was considered as a result of monodentate interaction,49 where only one of the two oxygen atoms in a given COO group was responsible for the interactions with the brucite-like layers. For each intercalate in our case, the asymmetric vibration υas (COO) is observed at 1630 cm-1 (Figure 1). The ∆υ is calculated to be 246 cm-1 for Mg2.97Al-Ti(IV)TA2 and Co2.23Al-Ti(IV)TA2 and 265 cm-1 for Zn3.00Al-Ti(IV)TA2. The ∆υ larger than 204 cm-1, observed here in each case, agrees well with the monodentate interaction (Figure 4b) between the interlayer Ti(IV)TA2 anions and the brucite-like layers. The ∆υ increases in the order of Mg2.97Al-Ti(IV)TA2 ≈ Co2.23Al-Ti(IV)TA2 < Zn3.00Al-Ti(IV)TA2, consistent with the stronger interaction between Ti(IV)TA2 and Zn/Al brucite-like layer deduced above from the pKa of hydroxides. In the 13C CP/MAS NMR spectrum of Zn3.00Al-Ti(IV)TA2 (Figure 2d), the signal of coordinated C-O is observed at 184.8 ppm, downfield shifted in comparison to that for Mg2.97Al-Ti(IV)TA2 (Figure 2c). The shift is supposed to result from less Oδ- f Cδ+ electron transfer in Zn-O-H · · · O-CdO-Ti linkage than in Mg-O-H · · · OCdO-Ti, consistent with the stronger interaction of interlayer Ti(IV)TA2 with Zn/Al layer. Stronger interaction means less Oδf Cδ+ electron transfer and thus less electron density of the Ti center. Structural Stability. The thermal stability of interlayer titanium tartarte has been investigated using TG-MS (Figure 9A) and in situ XRD (Figure 9B) techniques. As can be seen from Figure 9A, released (m/z ) 18) numerously were the interlayer and absorbed water molecules in the temperature range of 380-493 K. The dehydroxylation (m/z ) 17) of the brucitelike layers occurred above 473 K, accompanied by the removal of water molecules from 493 K. The interlayer complex starts to decompose at 380 K as indicated by the dissociation of the butoxyl group (m/z ) 73, 74), and their profound decomposition of the interlayer complex takes place from 493 K. With increasing temperature, the fragments of the butoxyl group (m/z ) 73, 74, at 506 K), butyl (m/z ) 57, at 527 K), and methyl (m/z ) 15, at 548 K) are observed in succession. The dissociation of the carboxyl (m/z ) 45) and carboxylate (m/z ) 44) is observed at 561 and 574 K. The in situ XRD patterns



Figure 8. Proposed arrangement of interlayer Ti(IV)TA2 and tartrate anions (C cyan b, H small gray b, O red b, Ti dark blue b, MII light blue b, Al large gray b).

Figure 9. In situ mass profiles (A) and in situ XRD patterns (B) of titanium tartrate intercalated LDHs heated under nitrogen atmosphere.

of titanium tartrate intercalated LDHs are observed to change consistent with TG-MS results. In the temperature range of 298-473 K, the reflections characteristic of titanium tartarte intercalated LDHs stay almost unchanged with increasing temperature, meaning that the intercalated structure was well

preserved. But the interlayer spacing decreases from 1.61 to 1.42 nm at 373 K and further 1.32 nm at 473 K. The observed contraction of interlayer spacing is dominantly due to the removal of the interlayer and absorbed water molecules at this temperature range. Above 573 K, the intercalated structure is observed to disappear as indicated by the absence of (00l) diffractions. On further heating above 623 K, the absence of characteristic (110) reflection indicates the disappearance of the layer structure, as a result of the continuous dehydroxylation of the brucite-like layers. MgO-Al2O3 phase emerges above 650 K. The Ti(IV)TA2 intercalated LDHs have been used as catalysts in the sulfoxidation of prochiral methyl phenyl sulfide (MPS) in MeOH/CH2Cl2 (V/V ) 1/1) with the input ratio of substrate/ Ti/H2O2 fixed at 20/1/1.1. As shown in Figure 10, the catalytic application has caused no visible change in the XRD patterns of either Ti(IV)TA2 or Ti(IV)TA intercalated LDHs. No reflections characteristic of CO32- intercalated LDH are observed as well. Ti leaching is also found to be less than 2%. That means the intercalated titanium tartrate anions are stable enough to resist the reverse exchange by ambient carbonate. In the FT-IR spectra of Ti(IV)TA2 intercalated LDHs (left), the bands assigned to CdO-Ti at 1637 and 1384 cm-1, C-O-Ti coordination at 1078 cm-1 are still observed after the catalytic reaction. For the Ti(IV)TA intercalated LDHs (right), the bands attributed to CdO-Ti are preserved at 1639 and 1379 cm-1, and the band for C-O-Ti coordination at 1074 cm-1. This means that the coordination structures of the interlayer titanium tartrate complexes have been well maintained after the catalytic reaction. Catalytic Activity and Selectivity in Sulfoxidation. Figure 11 shows the profiles of MPS conversion and methyl phenyl sulfoxide (MPSO) selectivity on the catalysts with different brucite-like layers. The catalytic sulfoxidation on Mg2.97AlTi(IV)TA2, Co2.23Al-Ti(IV)TA2, and Zn3.00Al-Ti(IV)TA gave similar chemical selectivity for MPSO, all above 90%. Elevating the reaction temperature from 273 to 298 K has significantly improved the conversion but hardly made adverse effects on the MPSO selectivity. The conversion of MPS and the selectivity of MPSO are summarized in Table 3. Among the catalysts investigated, Zn3.00Al-Ti(IV)TA2 exhibits better catalytic rate and higher conversion than Mg2.97Al-Ti(IV)TA2 and Co2.23Al-Ti(IV)TA2. The catalytic conversion of MPS on the latter two is similar. The catalytic activity could be well

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Figure 10. XRD patterns (top) and FT-IR spectra (bottom) of fresh (a) and spent (b) titanium tartrate intercalated LDHs.

TABLE 3: Oxidation of MPS Catalyzed by LDHs with H2O2 as the Oxidanta

a The heterogeneous reaction time was 9 h. The 100% conversions of homogeneous counterpart were achieved after 7 h at 273 K. Determined using GC-MS on a DB-5 column (poly(5% diphenyl-95% dimethylsiloxane), 25 m × 0.2 mm, 0.33 µm film thickness). c Determined by HPLC system with a chiral OB-H column (Daicel) and a n-hexane/isopropyl alchohol (V/V ) 80/20, or V/V ) 90/10). b

explained by the difference in the interaction of Ti(IV)TA2 with brucite-like layers. It was reported that oxidation of organosulfur compounds occurred through the electrophilic attack of activated peroxo complex to sulfide.50 Hence the catalytic activity in the sulfoxidation is supposed to increase with increasing electron

deficiency in the Ti centers. The above FT-IR and NMR characterizations demonstrate that the electronic density of the Ti center in Zn3.00Al-Ti(IV)TA2 is less than that in Mg2.97AlTi(IV)TA2 and Co2.23Al-Ti(IV)TA2 because the Zn/Al layers impose a stronger interaction on the interlayer Ti(IV)TA2, which


J. Phys. Chem. C, Vol. 114, No. 41, 2010 17827 previous work,28 the enhancement of asymmetric selectivity in the heterogeneous system originated from the confinement effects of the LDH interlayer space, which was proved to be the region where the catalytic reaction took place. The maximum improvement of enantioselectivity by intercalation was observed on Mg2.97Al-Ti(IV)TA2 with interlayer spacing of 1.61 nm, while the minimum increase in ee value was observed on Co2.23Al-Ti(IV)TA2 with interlayer spacing of 1.78 nm. Although Zn/Al layer imposes stronger interaction on the interlayer Ti(IV)TA2 than Mg/Al and Co/Al layers, the elevation of ee induced by Zn3.00Al-Ti(IV)TA2 is medium. The enantioselectivity improvement shows no immediate dependence on the host-guest interaction. In view of inferior catalytic activity and enantioselectivity of Co2.23Al-Ti(IV)TA2, Co/Al intercalates seem not to be a good catalyst for the asymmetric sulfoxidation. To confirm the catalytic induction of the interlayer Ti(IV)TA2 complex, a control experiment has been carried out on the supernatant after removing the Mg2.97Al-Ti(IV)TA2 by centrifugation. The solid is separated from the reaction mixture when the reaction time is 0.5 h, where a conversion of 3% and an ee of 29% are reached. The supernatant is stirred at the same reaction temperature for another 8.5 h. It is found that in 8.5 h the conversion of MPS increases to only 7% while the ee reduces to 11%. The result indicates that it is the interlayer Ti(IV)TA2 that contributes to both enantioselectivity and conversion in the heterogeneous catalysis. It also means that the Ti(IV)TA2 anions have been preserved in the interlayer regions in the catalytic reaction. Conclusions

Figure 11. Variation of the conversion of MPS (solid line) and selectivity of MPSO (dashed line) catalyzed by Mg2.97Al-Ti(IV)TA2 (9), Zn3.00Al-Ti(IV)TA2 (2), and Co2.23Al-Ti(IV)TA2 (0) at (A) 273 K and (B) 298 K.

well accounts for their difference in the catalytic activity. Meanwhile, visible difference has been observed in the sulfoxidation conversions on the Zn3.00Al-Ti(IV)TA2 and Co2.23Al-Ti(IV)TA2 that have similar interlayer spacing. So it can be concluded that the host-guest interaction is the prominent factor influencing the catalytic activity of the interlayer Ti(IV)TA2. Negligible (273 K) or below 10% (298 K) conversion has been accomplished on Mg2.98Al-CO32-, Zn3.00Al-CO32-, and Co3.17Al-CO32-, indicating the catalytic roles of interlayer Ti(IV)TA2 complex. The lower MPS conversions on the intercalates than on pristine Ti(IV)TA2 complex in homogeneous system are supposed to result from the diffusion limitation of the reactants into the interlayer spaces. The asymmetric induction of Ti(IV)TA2 intercalated LDHs in the catalytic sulfoxidation is also shown in Table 3. Comparing the ee on the heterogeneous catalysts with their homogeneous counterpart, it can be concluded that the chiral induction of Ti(IV)TA2 complex has been visibly improved by the intercalation. Only a trace of sulfone is formed at 273 K, meaning that the enantioselectivity of sulfoxide is a direct result of the asymmetric sulfoxidation rather than the kinetic resolution of sulfoxide by overoxidation of one sulfoxide enantiomer. The absolute configuration of sulfoxide has been determined by circular dipolarimetry to be R-configuration. According to our

The titanium tartrate complex has been intercalated into the interlayer space of Mg/Al, Zn/Al, and Co/Al LDHs in an interdigitated bilayer arrangement. The interlacement of interlayer titanium tartrate complex has been modulated by adjusting the charge density of unit brucite-like layer, and the arrangement density of the interlayer titanium tartrate complex in the b a or b b b direction along the (a b, b) plane has been tuned by adjusting its charge occupancy. The chemical composition of the brucitelike layers as hosts hardly influences the interlayer arrangement of titanium tartrate anions as guests but makes an impact on the host-guest interaction. Zn/Al layer imposes stronger interaction on the interlayer titanium tartrate anions than Mg/ Al and Co/Al layers, resulting in less electron density of the Ti center. The stronger the interaction between titanium tartrate and brucite-like layers, the higher the catalytic activity in the asymmetric sulfoxidation. The intercalation of titanium tartrate complex improves the enantioselectivity, wherein the interlayer spacing imposes more positive effect on the enantioselectivity rather than the host-guest interaction. The structures of titanium tartrate intercalated LDHs are found to be stable enough to endure the catalytic sulfoxidation. Acknowledgment. The authors are grateful for the financial support from NSFC and the “973” Program (2009CB939802). The XAS experiments were supported by National Synchrotron Radiation Laboratory in Hefei, China. References and Notes (1) Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974. (2) Pitchen, P.; Duiiach, E.; Deshmukh, M. N.; Kagan, H. B. J. Am. Chem. Soc. 1984, 106, 8188. (3) Scettri, A.; Bonadies, F.; Lattanzi, A.; Senatore, A.; Soriente, A. Tetrahedron: Asymmetry 1996, 7, 657. (4) Farrall, M. J.; Alexis, M.; Trecarten, M. NouV. J. Chim. 1983, 7, 449.

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(5) Canali, L.; Karjalainen, J. K.; Sherrington, D. C.; Hormi, O. Chem. Commun. 1997, 123. (6) Choudary, B. M.; Valli, V. L. K.; Prasad, A. D. J. Chem. Soc., Chem. Commun. 1990, 1186. (7) Xiang, S.; Zhang, Y. L.; Xin, Q.; Li, C. Angew. Chem., Int. Ed. 2002, 41, 821. (8) Notestein, J. M.; Katz, A. Chem.sEur. J. 2006, 12, 3954. (9) Fraile, J. M.; Garcıá, J. I.; Mayoral, J. A.; Roldań, M. Org. Lett. 2007, 9, 731. (10) Li, C.; Zhang, H.; Jiang, D.; Yang, Q. Chem. Commun. 2007, 547. (11) Corma, A.; Garcıá, H.; Sastre, G.; Viruela, P. M. J. Phys. Chem. B. 1997, 101, 4575. (12) Kidder, M. K.; Britt, P. F.; Zhang, Z.; Dai, S.; Hagaman, E. W.; Chaffee, A. L.; Buchanan, A. C. J. Am. Chem. Soc. 2005, 127, 6353. (13) Goettmanna, F.; Sanchez, C. J. Mater. Chem. 2007, 17, 24. (14) Jones, M. D.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Lewis, D. W.; Rouzaud, J.; Harris, K. D. M. Angew. Chem., Int. Ed. 2003, 42, 4326. (15) Yang, H.; Zhang, L.; Zhong, L.; Yang, Q.; Li, C. Angew. Chem., Int. Ed. 2007, 46, 6861. (16) Raja, R.; Thomas, J. M.; Jones, M. D.; Johnson, B. F. G.; Vaughan, D. E. W. J. Am. Chem. Soc. 2003, 125, 14982. (17) Pe´rez, C.; Pe´rez, S.; Fuentes, G. A.; Corma, A. J. Mol. Catal. A: Chem. 2003, 197, 275. (18) Zhang, H.; Xiang, S.; Li, C. Chem. Commun. 2005, 1209. (19) Zhang, H.; Zhang, Y.; Li, C. J. Catal. 2006, 238, 369. (20) Caps, V.; Paraskevas, I.; Tsang, S. C. Chem. Commun. 2005, 1781. (21) Raynor, S. A.; Thomas, J. M.; Raja, R.; Johnson, B. F. G.; Bellb, R. G.; Mantlec, M. D. Chem. Commun. 2000, 1925. (22) McMorn, P.; Hutchings, G. J. Chem. Soc. ReV. 2004, 33, 108. (23) Fraile Jose., M.; Garcıá J., I.; Mayora, J. A. Chem. ReV. 2009, 109, 360. (24) Khan, A. I.; O’Hare, D. J. Mater. Chem. 2002, 12, 3191. (25) Bhattacharjee, S.; Anderson, J. A. Chem. Commun. 2004, 554. (26) Choy, J. H.; Kwak, S. Y.; Park, J. S.; Jeong, Y. J.; Portier, J. J. Am. Chem. Soc. 1999, 121, 1399. (27) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. (28) Shi, H.; Yu, C.; He, J. J. Catal. 2010, 271, 79. (29) Zhao, Y.; Li, F.; Zhang, R.; Evans, D. G.; Duan, X. Chem. Mater. 2002, 14, 4286. (30) Zhong, W.; He, B.; Li, Z.; Wei, S. J. China UniV. Sci. Technol. 2001, 31, 328.

Shi et al. (31) Finn, M. G.; Sharpless, K. B. J. Am. Chem. Soc. 1991, 113, 113. (32) Potvin, P. G.; Fieldhouse, B. G. Tetrahedron: Asymmetry. 1999, 10, 1661. (33) Potvin, P. G.; Bianchet, S. J. Org. Chem. 1992, 57, 6629. (34) Farges, F.; Brown, G. E., Jr.; Rehr, J. J. Phys. ReV. B. 1997, 56, 1809. (35) Teo, B. K. EXAFS: basic principles and data analysis; SpringerVerlag: Berlin, 1985. (36) Babonneau, F.; Doeuff, S.; Leaustic., A.; Sanchez, C.; Cartier, C.; Verdauer, M. Inorg. Chem. 1988, 27, 3166. (37) Doeuff, S.; Dromzee, Y.; Taulelle, F.; Sanchez, C. Inorg. Chem. 1989, 28, 4439. (38) Moran, P. D.; Rickard, C. E. F.; Bowmaker, G. A.; Cooney, R. P. Inorg. Chem. 1998, 37, 1417. (39) Pedersen, S. F.; Dewan, J. C.; Eckman, R. R.; Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 1279. (40) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic, G. J. Phys. Chem. 1994, 98, 4125. (41) Gregor, B.; Lytle, F. W.; Sandstrom, D. R.; Wong, J.; Schultz, P. J. Non-Cryst. Solids. 1983, 55, 27. (42) Emili, M.; Incoccia, L.; Mobilio, S.; Fagherazzi, G.; Guglielmini, M. J. Non-Cryst. Solids. 1985, 74, 129. (43) Zhang, W.; Fro¨ba, M.; Wang, J.; Tanev, P. T.; Wong, J.; Pinnavaia, T. J. J. Am. Chem. Soc. 1996, 118, 9164. (44) Park, J. H.; Yang, J. H.; Yoon, J. B.; Hwang, S. J.; Choy, J. H. J. Phys. Chem. B. 2006, 110, 1592. (45) Narlikar, G. J.; Gopalakrishnan, V.; Mcconnell, T. S.; Usman, N.; Herschlag, D. Proc. Natl. Acad. Sci. 1995, 92, 3668. (46) Dahm, S. C.; Derrick, W. B.; Uhlenbeck, O. C. Biochemistry 1993, 32, 13040. (47) Vincent, J. B.; Crowder, M. W.; Averill, B. A. Trends Biochem. Sci. 1992, 17, 105. (48) Prevot, V.; Forano, C.; Besse, J. P.; Abraham, F. Inorg. Chem. 1998, 37, 4293. (49) Kakihana, M.; Nagumo, T.; Okamoto, M.; Kakihana, H. J. Phys. Chem. 1987, 91, 6128. (50) Chica, A.; Gatti, G.; Moden, B.; Marchese, L.; Iglesia, E. Chem.sEur. J. 2006, 12, 1960.

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