Article pubs.acs.org/Langmuir

Synthesis of Helical and Supplementary Chirally Doped PMO Materials. Suitable Catalysts for Asymmetric Synthesis Rafael A. García-Muñoz,*,† Victoria Morales,† María Linares,‡ and Beatriz Rico-Oller† †

Department of Chemical and Environmental Technology and ‡Department of Chemical and Energy Technology, ESCET, Rey Juan Carlos University, C/Tulipán s/n, 28933, Móstoles, Madrid, Spain S Supporting Information *

ABSTRACT: Exciting helical mesoporous organosilicas including supplementary chirally doped moieties into their spiral walls were one-pot successfully synthesized with good structural order for, to the best of our knowledge, the first time. This onestep direct synthesis of helical chirally doped periodic mesoporous organosilica (PMO) materials was carried out by combination of a tartrate-based bis-organosilicon precursor with tetraethyl orthosilicate (TEOS) and two surfactants, cetyltrimethylammonium bromide and perfluoroctanoic acid (CTAB and PFOA). For comparison purposes, a conventional two-step postsynthetic grafting methodology was carried out. In this method, the chiral tartrate-based moieties were grafted onto the helical silica mesoporous materials previously prepared by the dual-templating approach (CTAB and PFOA). The chirally doped materials prepared by both methodologies exhibited helical structure and high BET surface area, pore size distributions, and total pore volume in the range of mesopores. Solid-state 13C and 29Si MAS NMR experiments confirmed the presence of the chiral organic precursor in the silica wall covalently bonded to silicon atoms. Nevertheless, one-pot direct synthesis led to a greater control of surface properties and presented larger incorporation of organic species compared with the two-step postsynthetic methodology. To further prove the potential feasibility of these materials in enantiomeric applications, Mannich diastereoselective asymmetric synthesis was chosen as catalytic test. In the case of the one-pot PMO material, the rigidity of the chiral ligand backbone provided by its integration into the inorganic helical wall in combination with the steric impediments supplied by the twisted geometry led to the reagents to adopt specific orientations. These geometrical constrictions resulted in an outstanding diastereomeric induction toward the preferred enantiomer.

1. INTRODUCTION The development of ordered solids materials with controllable structures and systematic tailoring pore architecture is required for the technical advances in fields such as adsorption, separation, catalysis, drug delivery, nanotechnology, or sensors.1−3 In the early 1990s the first kinds of mesoporous materials families were synthesized: M41S and FSM-n.3−6 Over the past two decades, various mesoporous structures have been synthesized, the most important being HMS (Hexagonal Mesoporous Solids), MSU (Michigan State University), and SBA-15 (Santa Barbara Amorphous) materials. All these materials are characterized by employing a single surfactant in their syntheses. Nonetheless, actual research lines are focused on the use of surfactant mixtures, obtaining structures other than those described above due to electrostatic interactions, van der Waals forces, and hydrogen bonds between the costructure directing agents.7 This is the case of siliceous mesostructured materials with helical structurethe approach which will be used in this work. The synthesis of helical mesoporous silicas (HelMS) arises from the interest to acquire artificial helices that mimic natural systems with spiral structures (such as biological molecules of DNA and proteins). Instead of having straight channels as in © 2014 American Chemical Society

the case of MCM-41, these hexagonal shape materials are characterized by having twisted channels, leading to materials that can be considered warped version of the conventional silica mesoporous materials.4 The field of helical mesoporous silicas is an emerging but very active area of research. Thus, several investigations based upon either chiral surfactants8−16 or a mixture of achiral surfactants9,17−23 describe how to induce helical character into the resulting material. In the case of helical materials, there is still controversy to explain the formation mechanism of these helical channels. Yang et al. proposed an interfacial interaction mechanism where the surface free energy reduction is the driving force for the spontaneous formation of the spiral morphology.4,9 In the past years, the heterogeneous catalysts derived from tartaric acid, cinchonidine, or Binol have contributed to the development fine chemicals production processes.24,25There are many ways to introduce catalytically chiral active species in mesoporous silica.2 These strategies include the postsynthetic incorporation methods that normally lead to a nonhomogeReceived: September 26, 2013 Revised: January 5, 2014 Published: January 7, 2014 881

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neous surface coverage.6,26 Also, another widely used the and most advantageous method to synthesize functionalized materials of different morphologies is the co-condensation of chiral active species during the synthesis of the materials. This straightforward procedure enables to synthesize the commonly called periodic mesoporous organosilicas (PMOs). The cocondensation of tetraalkoxysilanes and bis-organosilanes in the presence of surfactants gives rise to materials with organic groups covalently anchored and homogeneously distributed into the pore walls.27 The introduction of chiral centers in PMOs materials opens new applications such as chiral chromatography, chiral separations, and asymmetric catalysis. The first pioneering study of chiral PMOs with a catalytic application, denoted ChiMOs (chiral mesostructured organosilicas), was published by Corma et al.,28 who successfully incorporated vanadyl−salen complexes as bridging components within the silica framework. Later, Polarz and Kuschel in 2006 attained the synthesis of chiral hexagonal mesoporous structures by means of organic precursors under basic conditions with CTAB as silica directing structure.29 A similar methodology to obtain pure chiral PMOs was further developed by Thomas et al.,30 obtaining a PMO material with chiral amines incorporated into the framework. In 2008, another interesting approach to generate PMOs from 100% bissilylated chiral bridged precursors was published by Inagaki et al.31 One of the homogeneous catalysts with greater repercussion on asymmetric synthesis is the tartaric acid derivative, known as Sharpless catalyst. Since its discovery in 1980 by K. B. Sharpless (Nobel Prize in Chemistry, 2001), it has been used in a large number of applications in the industry, implemented for instance to obtain pharmaceutical gastrointestinal protector Omeprazole by enantioselective sulfoxidation of thioanisole or to achieve antidepressant Fluoxetine Prozac active component.32 Despite presenting a high catalytic activity and high stereoselective capacity, its organometallic structure is extremely unstable, which hampers the further industrial application. Thereby, a considerable amount of research is focused to switch this homogeneous catalyst by the heterogeneous counterpart. In this context, Garcia et al. synthesized chiral PMOs materials with MCM-41 likestructure, incorporating in its three-dimensional network the Sharpless catalyst complex following a two-step synthesis methodology,33 and recently, the same group published the immobilization of tartaric derivative following a one-step direct synthesis.34,35 Considering the above, the pursued aim of this research was to one-pot synthesize helical PMOs materials with supplementary chirally dopant moieties incorporated into the helical wall for, to the very best of our knowledge, the first time. This gave rise to materials with uniformly twisted rodlike with hexagonal cross section and chiral moieties added along the wall spiral channel. Furthermore, these helical chirally doped PMOs were tested in the Mannich diasteroselective reaction, providing an outstanding enantioactivity. Therefore, these materials seem to induce an extra activity that could be highly beneficial not only in heterogeneous asymmetric synthesis but also in different applications, such as chiral separation, and even in drug delivery systems to maintain a sustained enantio-active pharmaceutical substance concentration in the bloodstream for an adequate therapy.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Cetyltrimethylammonium bromide (SigmaAldrich), perfluorooctanoic acid (Sigma-Aldrich), sodium hydroxide (Scharlab), tetraethyl orthosilicate (TEOS, Aldrich), dimethylamine (Sigma-Aldrich), dimethyl tartrate (Sigma-Aldrich), N-methylaminopropyltrimethoxysilane (ABCR), toluene anhydrous (Scharlab), nbutylstannonic acid (Aldrich), n-hexane (Scharlab), diethyl ether (Scharlab), trifluoroacetic acid (Aldrich), hydrochloric acid (Scharlab), aniline (Sigma-Aldrich), benzaldehyde (Sigma-Aldrich), cyclohexane (Sigma-Aldrich), and phosphoryl chloride (Aldrich). 2.2. Materials. Synthesis of the Helical Mesoporous Material (HelMS). The synthesis of siliceous helical mesoporous material with hexagonal structures was based on the method described by Yang and co-workers,9 under basic conditions using cetyltrimethylammonium bromide (CTAB) and perfluorooctanoic acid (PFOA) as cotemplates. A typical synthesis was performed as follows: CTAB (0.26 g, 93.97 mmol) was dissolved in deionized water (125 g, 6.9 mol), stirring at room temperature for 30 min. Then, 0.91 mL of NaOH (2 M) and 0.026 g of PFOA were added separately into the solution. The temperature of the solution was raised and kept at 80 °C for 2 h. To this solution, 1.74 mL of tetraethyl orthosilicate (TEOS) was added. The mixture was continuously stirred for an additional 2 h. The resulting products were collected by filtration and dried at room temperature (HelMS(0.1)). The templates were removed by calcination at 550 °C for 5 h (1.8 °C/min rate). Synthesis of the Reference Material (MCM-41). The synthesis of MCM-41 material was accomplished by using the method described by Lin and co-workers,6 detailed in the Supporting Information Scheme S1. Synthesis of Tartrate-Based Bis-silane. The synthesis of bissilylated tartramide chiral compound was accomplished by using the method described by Garciá and co-workers33 and shown in Scheme S2 (Supporting Information). Direct Synthesis of Helical Chiral Periodic Mesoporous Organosilicas. The syntheses of chiral PMO materials were accomplished by dissolving CTAB (0.26 g, 93.97 mmol) in deionized water (125 g, 6.9 mol) and stirring until complete dissolution. Then, 0.91 mL of NaOH (2 M) and PFOA (0.026 g, 0.06 mmol) were added separately into the solution. The temperature of the solution was raised and kept at 80 °C for 2 h. To the transparent resultant solution, TEOS and the tartramide precursor were added separately by dropping. The materials were prepared fixing molar ratio tartramide to TEOS of 15:85. The tartramide precursor (0.42 g) was added separately at different prehydrolysis times (0, 30, and 60 min), resulting in different materials (HelMS(0.1)SD0, HelMS(0.1)SD30, and HelMS(0.1)SD60, respectively). Stirring is continued for 2 h, starting counting the time when the bis-silane is added. The resultant materials were then recovered by filtration and air-dried. As-made materials were treated with an ethanolic solution of hydrochloric acid (0.5 N) (1.5 g of as-made material per 400 mL of acid solution) for the removal of the surfactant. Finally, the solid is recovered by vacuum filtration. Grafting of Tartrate-Based Bis-silane on SiO2 Materials. For comparison, a two-step postsynthetic functionalization of a silica helical sample with the chiral bis-silylated tartramide was carried out. This method relies on the reaction between the inorganic silica silanol groups with the chiral bis-silane moieties. For this, after calcination, 0.5 g of helical silica material is weighed in a three-necked flask, and it is activated by the vacuum degassing process at 130 °C for 24 h under stirring. After that, 150 mL of toluene was added to the flask (fitted with a condenser), and the mixture was stirred for 1 h to get a homogeneous dispersion. The tartrate-based bis-silane was added (50% w/w, 0.25 g), heating the mixture to reflux and stirring at 110 °C for 24 h. The solid material was recovered by vacuum filtration, washed with toluene, and dried at room temperature for 2 days. This procedure was carried out over the pure silica helical material (HelMS(0.1)GR) and over the mesoporous reference material (MCM-41GR). 2.3. Characterization Techniques. The prepared materials were characterized by means of different analytical techniques. Thus, nitrogen adsorption−desorption isotherms were collected at 77 K 882

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using a Micromeritics TriStar 3000 unit. The samples were previously outgassed in N2 flow at 125 °C for 8 h before analysis. Surface specific areas were calculated using the BET method. Pore size distributions were determined by the BJH method using the KJS correction and assuming cylindrical pore geometry. The total pore volume was assumed as the one recorded at P/P0 = 0.995. X-ray powder diffraction patterns were collected on a Philips X’Pert MPD diffractometer fitted with an accessory for the operation at low angle values. XRD analyses were recorded using the Cu Kα line (λ = 1.54 Å) in the 2θ range from 0.5° to 5° with a step size of 0.03°. NMR spectra for liquid samples were recorded on a Varian Mercury 400 MHz spectrometer. Chemical shifts were reported in parts per million (ppm), in reference to the residual proton signals from the deuterated solvents. Solid-state CPMAS 13C and MAS 29Si NMR experiments were performed on a Varian Infinity 400 MHz spectrometer fitted with a 9.4 T magnetic field. These nuclei resonate at 100.53 and 79.41 MHz, respectively. An H/X 7.5 mm MAS probe and ZrO2 rotors spinning at 6 kHz were used. On CP experiments, the cross-polarization time was determined to guarantee the total proton polarization verifying the Hartmann− Hann condition. In addition, to allow an accurate quantification of silicon groups, 29Si NMR spectra using one pulse sequence were also obtained. For 13C acquisition, π/2 pulse, number of scans, repetition delay, and contact time were 4.25 μs, 2000 scans, 3 s, and 1 ms, respectively. The 29Si experiments were performed for 3000 scans, π/2 pulse of 3.5 μs, and 15 s of repetition time. 13C and 29Si chemical shifts were externally referenced to adamantane and tetramethylsilane, respectively. Transmission electron microscopy (TEM) images were obtained by means of a Philips Tecnai 20 microscope operated at 200 kV, with thermoionic gun and LaB6 filament, 2.7 Å resolution, and ±70° tilt of the sample. For TEM measurements, the samples were prepared by dispersing the powder samples in acetone, after that they were dispersed and dried on carbon film on a Cu grid. Scanning electron microscopy (SEM) images were obtained on a XL30 ESEM Philips microscope with electronic energy dispersive spectrometer (EDS) operated at 20 kV. The samples were also dispersed in acetone and subsequently depositing a few drops of mixture on a sample holder, being coated with a gold layer to improve the conductivity of the sample. 2.4. Catalytic Tests. In a conventional stirred glass, 39.4 μL of aniline, 75 μL of benzaldehyde, 3 μL of POCl3, and 28 mg of the solid material were combined in 5 mL of toluene at room temperature for 30 min. Then 472 μL of cyclohexane was added, and the mixture was stirred at room temperature for 48 h.36 Reaction products were identified and quantified by gas chromatography, using a GC-3900 Varian chromatograph equipped with a CP-Chirasil-Dex CB #CP7502 capillary column and flame ionization detector (FID).

S3 (Supporting Information) summarizes the mesostructured nanoparticles prepared in this work. The chiral bis-organosilane was first prepared following an strategy previously published,35 in which (N-methyl-3-aminopropyl)trimethoxysilane was reacted with the chiral protected dimethyl tartrate, following the procedure shown in Scheme S1 (Supporting Information), to give the bis-silylated tartramide chiral ligand. Low-angle XRD patterns of calcined pure silica helical mesoporous material HelMS(0.1) and the MCM-41 SiO2 material used as references are shown in Figure 1. Well-

Figure 1. Low-angle XRD patterns recorded for the pure and helical MCM-41 and hybrid organic−inorganic helical chiral MCM-41-type materials.

resolved peak in the range of 2θ 2°−2.5° can be indexed in both samples, based on a two-dimensional hexagonal symmetry MCM-41 type. Regarding the position of the main XRD signals, the helical sample presented the reflections at a slightly higher 2θ value, indicating the reduction of the pore size diameter compared to the MCM-41 SiO2 reference sample. The tartrate-based mesostructured samples prepared through the two-step grafting procedure (sample HelMS(0.1)-GR and MCM-41-GR, respectively) show the typical diffraction pattern of a mesostructured solid, the main diffraction signal (d100) being less intense in the samples chirally doped compared to the pure silica materials. Nevertheless, these hybrid organic− inorganic materials preserve the mesostructured ordering of the former starting mesostructured phase. However, as frequently stated in the literature,34 in the postsynthetic grafting routes the incorporation efficiency of the organic functionality to the final material is usually rather low. For this reason, the simultaneous co-condensation (one-pot synthesis) of organoalkoxysilane and the corresponding tetraalkoxysilanes silica precursors is an alternative that leads to higher amounts of catalytically active organic moieties incorporation. Consequently, the bis-silylated tartramide chiral precursor, previously synthesized, was used in combination with TEOS, and CTAB and PFOA as templates, to prepare helically and chirally doped PMO MCM-41-type materials. Accordingly, with this methodology the chiral organic functionality is incorporated into the silica walls framework, since the built-organic complex is a bis-silane. Helical PMO’s-

3. RESULTS 3.1. Textural Properties Determination of the Helical Chirally Doped PMO Materials. The current work develops an interesting and easy approach to synthesize helical PMOs materials with supplementary chirally doped moieties incorporated into the twisted mesoporous channels. The two different features included in the mesostructured materials have a positive repercussion on the enantiocatalytic properties of these materials. The helical PMOs organosilicas with tartaric chiral ligands incorporated into the framework were obtained following a one-pot co-condensation procedure using a dual templating methodology (PFOA/CTAB). The TEOS prehydrolysis time has been studied in order to achieve the adequate textural properties, since it is a critical parameter to integrate the chiral bis-silane into the PMOs pore walls. For comparison purposes, pure SiO2 MCM-41 materials and pure silica helical materials (HelMS(0.1)) were prepared as references. Subsequently, in a second step, the chiral tartrate bis-silane was grafted to the silanol groups of the linear and helical reference supports. Table 883

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Figure 2. (a) N2 adsorption−desorption isotherms and (b) pore size distribution.

with the chiral bis-organosilane. Although the silica framework is not readily developed until TEOS has been added to the synthesis medium, the coverage of micelles is not complete in early stages, and thus the tartrate-based bis-silane can migrate through the former silica layer to interact with the surfactant. Therefore, larger prehydrolysis times lead to somewhat lower incorporation efficiency of the chiral ligands, as it will be further stated from data collected from 29Si MAS NMR. Sample HelMS(0.1)SD60, prepared with a TEOS prehydrolysis time of 60 min, displays lower concentration of chiral bis-silane groups than the other materials, probably because the access of the chirally dopant precursor to the micelles is significantly hindered. Figure 2a shows the N2 adsorption−desorption isotherms and pore size distribution for the synthesized materials. Accordingly to the IUPAC classification, the nitrogen adsorption−desorption curves lie in the group of type IV isotherms, which are typical of mesostructured materials, as displayed in Figure 2. The sharp adsorption step detected around P/P0 = 0.25−0.40, corresponding to the capillary condensation of nitrogen in uniform pores, evidence the formation of well-structured solid materials with narrow pore sizes distributions. This result agrees with the tight mesopores diameter range from 20 up to 30 Å, obtained from the BJH analysis, represented in Figure 2b. As far as the grafting materials, the adsorption−desorption isotherms evidence that the materials’ mesostructure is likewise preserved after the introduction of chiral dopant moieties. The step in the

type material, with the tartramide chiral moiety integrated in the silica spiral walls, were hence synthesized. Based on the optimization previously done,8 the ratio PFOA:CTAB were fixed in 0.1 for every sample. Regarding the ratio chiral bissilane/TEOS, Garcia et al. studied this parameter in the synthesis of linear chiral PMOs samples, reaching an optimum value when the molar ratio TEOS/bis-silylated chiral precursor was 85:15. N2 adsorption/desorption analysis suggested the formation of well-structured solid materials for samples with molar ratios of TEOS/chiral precursor higher than 85:15. Greater loadings of the chiral dopant functionality leaded to structurally distorted materials.33 On the basis of this result, a series of helical PMOs materials were prepared with the same amount of TEOS related to chiral moiety (85:15). Later, modification of the TEOS prehydrolysis time allows studying its effect over the helical formation and chiral bis-silane incorporation efficiency, since the hydrolysis of the TEOS alkoxide groups occurs simultaneously to the condensation of silicon species, though at different rates. The mesoporous structure formation was ascertained by XRD as shown in Figure 1. These chiral PMOs materials preserve the periodic mesostructure, concluding that the higher the TEOS prehydrolysis time the more ordered materials. Additionally, the reflection peaks shifted to higher angles, and remarkably increasing the main signal intensity. Previously to the addition of the chiral bis-silane generates a film of silica around the micelles is generated. This feature confers some additional stability against the strong interaction 884

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Table 1. Textural Properties for the Pure SiO2 and Hybrid Mesoporous Materials

a

material

SBETa (m2/g)

Dporeb (Å)

Vporoc (cm3/g)

d100d (Å)

a0e (Å)

epf (Å)

MCM-41 HelMS(0.1) MCM-41 GR HelMS(0.1) GR HelMS(0.1)SD 0 HelMS(0.1)SD 30 HelMS(0.1)SD 60

1018.6 751.9 632.1 530.4 636.8 719.4 883.5

24.7 21.5 22.7 20.1 34.6 26.6 26.8

0.59 0.54 0.46 0.36 0.77 0.83 0.94

40.8 40.6 40.4 40.6 47.6 40.8 40.8

47.1 46.9 46.6 46.9 55.0 47.1 47.1

22.4 25.4 23.9 26.8 20.4 20.5 20.3

BET method. bPore diameter (BJH). cTotal pore volume P/P0 = 0.975. dInterplane distance d100. ea0= 2d100/31/2. fWall thickness: ep = a0 − Dpore.

Figure 3. Images of pure SiO2 material: (a, b) MCM-41 SEM and TEM images, (c) HelMS(0.1) SEM images, (d−f) HelMS(0.1) TEM images.

material prepared with 60 min. In the case of sample without prehydrolysis time (HelMS(0.1)SD-0) the BET surface area and pore volume are around 600 m2/g and 0.77 cm3/g, respectively, i.e., 34% and 20% less BET surface area and pore volume. However, all the materials synthesized in this study preserved the mesoscopic structure. The more significant textural properties of these materials are summarized in Table 1. 3.2. Study of the Helical Chirally Doped PMO Materials Morphology. TEM and SEM analyses were carried out to study the morphology and pore architectures of the pure silica samples (Figure 3). MCM-41 pure silica (Figure 3a,b) synthesized by conventional procedure presents regular and stable hexagonal mesostructure. Related to the silica sample prepared with dual templating methodology using PFOA and CTAB as surfactants, HelMS(0.1) material, the SEM image of the product after the surfactants extraction (Figure 3c), indicates that nanoparticles with twisted hexagonal rod-like framework are formed.

isotherms adsorption branch remains almost in the same position, characteristic of MCM-41-type materials. However, the chiral moieties incorporation provokes a remarkably change on the textural properties, as consequence of their attachment to the inorganic silanols groups, evidenced by the pore volume and surface area decrease reflected in Table 1. Regarding the one-pot helical chirally doped PMO materials, it is significant the deep modification in the textural properties as the TEOS prehydrolysis time is modified, as summarized in Table 1. The higher the TEOS prehydrolysis time, the larger the surface area, pore volume, and pore diameter. The sample prepared without prehydrolysis time shows a broad and less intense pore size distribution, typical of less ordered mesostructured materials. Although the width of the pore size distribution seems not to be greatly modified, the area below the pore size distribution, corresponding to the total pore volume, is remarkably affected when increasing the TEOS prehydrolysis time, reaching values BET surface areas and pore volume near 900 m2/g and 0.94 cm3/g, respectively, for 885

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Figure 4. TEM images of hybrid organic and inorganic helical mesostructured materials.

theless, after 30 min of TEOS prehydrolysis time, materials with helical rod-like shape with diameter around 50 nm and lengths higher than 400−600 nm were obtained. The spiral channels along the length of the rod can also be observed in these samples, and accordingly it was proved the successfully synthesis of helical mesoporous PMO materials, with the distinctiveness of external chirally doped species incorporated into the spiral channels, as further on evidenced. 3.3. Quantification and Silicon Environment Analysis. To assess the incorporation of the chiral precursor into the inorganic silica framework, solid-state 29Si MAS NMR was performed (Figure 5). The spectra corresponding to the materials synthesized under both approaches, postgrafting and one-pot helical PMO materials, display the presence of T silicon species, with NMR signals between −45 and −65 ppm. These signals prove the incorporation of the chiral organic species into the Si−O−Si framework. The spectrum of hybrid organic−inorganic samples displayed two regions of major intensity, centered at about −100 to −110 and about −60 ppm. These two regions correspond to Si(−O−)4 and RSi(−O−)3 species, respectively, where R is the chiral tartrate-based organic group. The −100 to −110 ppm pattern is composed of at least three contributions, at about −92, −100, and −110 ppm, which correspond to (SiO)2Si(OR′)2 (Q2), (SiO)3−Si(OR′) (Q3), and (SiO)4Si (Q4) species, respectively, where R′ = Et or H. The lower-shielding pattern is formed by different signals at about −55 and −65 ppm, dealing with species containing the chiral organic groups anchored to the silanols groups, RSi(OSi)n (Tn).

The statistical analysis of the high-resolution images allows to estimate the amount of helical nanorods domains, as well as the density object (number of particles per/μm2), included in Supporting Information (Table S4). The analysis of 5 images with more than 500 particles permits to conclude that around 60−65% of the particles appeared as helices. Regarding the TEM images, it is possible to distinguish the rod-like fiber-type materials with spiral channels along the long axis of the rods. The magnification of these channels is also shown in Figure 3. Periodic fringes emerge perpendicularly to the fiber long axis, as indicated by double arrows in Figure 3e. Figure 3f displays even more clearly the fringes perpendicular to the main axis, denoted by the black arrows.8 The distance between adjacent fringes is around 110−125 nm, and the helical pitches of the nanorods are in a range values of 1−3 μm. In turn, the dual templating methodology is an adequate procedure that permits to obtain solid materials with helical channels. Figure 4 depicts the organic−inorganic materials TEM images obtained upon the addition of external chirally dopant agent by the both methodologies (postsynthesis two-step and one-pot PMO strategies). The helical structure, previously shown for the pure silica helical material (HelMS-(0.1)), preserved in the samples prepared by chiral organosilane grafting, as shown in Figure 4 for HelMS(0.1)-GR material. On the other hand, in one-pot methodology, the formation of the helical structure is clearly influence by the TEOS prehydrolysis time. In the samples synthesized without prehydrolysis step, the development of a near-spherical morphology with lower hexagonal ordered mesostructure and without helical channels is favored. Never886

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Regarding the helical PMOs samples, the spectra of two samples prepared with and without prehydrolysis time reveal that the chemistry of the surface in terms of silicon species population distribution is hardly modified. The spectra display a main region centered at around −55 ppm. Both samples exhibit the patterns corresponding to species containing the chiral organic precursor, named Tn groups. A larger amount of crosslinked T3, compared to T2 and T1 surface species, is detected, indicating the condensation reached for these chiral PMO materials is reasonably high. However, the amount of chiral bisorganosilane incorporated into the structure depends on the TEOS prehydrolysis time. The higher the prehydrolysis time, the lower the organic ligand integrated into the framework. The explanation is obvious; an increment of the prehydrolysis time facilitates more Si inorganic species condensation. The bisorganosilane loading calculated through solid-state NMR deconvoluted spectra matches well with the textural properties of these PMO materials, previously stated. Thus, the HelMS(0.1)SD60 material possesses higher BET surface area and pore volume than those of the PMO materials with shorter prehydrolysis time due to the lower bis-organosilane incorporation. Nevertheless, the catalytically active chiral organic moieties are enough allowed to be incorporated in the siliceous framework even at 60 min of prehydrolysis time, as shown in Figure 5. To get a more detailed characterization of the organic species incorporated into the framework of mesostructured helical PMO materials, solid-state 13C CP-MAS NMR was employed. Figure S5 (Supporting Information) depicts the spectrum recorded from HelMS(0.1)SD 60 material. The spectrum displays carbon resonances that have been assigned to the different functional groups of the silica-supported tartramide chiral species.35 The lower-shielding chemical shift at about 171 ppm (peak 5 in figure) corresponds to the carbonyl groups from the synthesized amide. At this chemical shift, one single peak is detected, meaning the symmetrical bis-tartramide is kept during the synthesis, without any bond cleavage. The signals centered at 58 and 38 ppm (peaks 3 and 4, respectively) can be assigned to −N−CH2 and −N−CH3, confirming the presence of the nitrogen atoms in the material. The presence of Si−CH2 and Si−CH2−CH2 could be identified at 8 and 19 ppm (peaks 1 and 2, respectively). The chemical shift at 71 ppm (peak 6) comes from the chiral carbon atoms bonded to hydroxyl groups, CH−OH. Thereby, the 13C NMR chemical shifts assignation confirms the integration of the chirally dopant agent into the helical rods walls, covalently bonded to the silicon atoms. 3.4. Chiral Catalytic Test. As we have mentioned before, one of the aims of this work resides in the potential enhancement of enantiocatalytic properties of the helical structure of the PMO materials by means of the integration

Figure 5. Solid state 29Si MAS NMR spectra of chiral helical materials obtained by grafting and one-step methodology (PMOs). Green and red lines show the deconvolution in Gaussians curves of T and Q species.

The quantification of these species is summarized in Table 2. For comparison purposes, the spectrum of the reference helical sample (HelMS(0.1)) was also including. In this spectrum, only Q species were observed, assigned to condensed and nontotally condensed silica species. With regard to the postsynthetic grafting route, the incorporation of the chiral organic functionality was confirmed by the presence of a shoulder centered at −60 ppm. Likewise, the one-pot methodology leaded to the incorporation of the chirally dopant agent, as evidenced by the clear signals centered at −60 ppm. However, the chiral organic incorporation is rather higher in the one-pot methodology, as displayed in Table 2. Additionally, in the onepot route, the chiral organic agent is incorporated into the silica walls framework, taking into account that the built-organic complex is a bis-silane. Thus, a helical PMO MCM-41 type material with potentially chiral catalytic moieties along the twisted mesoporous channels was successfully synthesized.

Table 2. 29Si MAS-NMR Quantification for Silicon and Organosilicon Species in Pure and Helical MCM-41 and Hybrid Organic−Inorganic Helical Chiral MCM-41-Type Materials material

Q4 (%)

Q3 (%)

Q2 (%)

T3 (%)

T2 (%)

organic loading (Tn/Tn + Qn) (%)

HelMS(0.1) MCM-41 GR HelMS(0.1) GR HelMS(0.1)SD 0 HelMS(0.1)SD 30 HelMS(0.1)SD 60

71.70 55.62 62.54 53.52 51.00 53.57

26.70 31.90 26.24 19.87 18.16 33.21

1.63 1.02 7.01 6.97 12.23 3.94

4.21 0.89 18.18 17.79 3.46

7.25 3.32 1.45 0.82 5.81

11.46 4.21 19.63 18.61 9.27

887

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Scheme 1. Catalytic Test: Mannich Reaction

ligand, were studied. The results displayed that an increase of ANTI β-amino ketone enantio-yield was achieved for these samples. Therefore, the dual nature of both helical channels morphology and chirally dopant species integrated into the inorganic framework along the helical rod contributed to reach higher values than those obtained featuring effects separately. Moreover, the better diastereoselectivities were obtained for the one-pot PMO materials. Thus, the preferred anti enantiomer product was obtained in a ratio anti:syn of 93:7 and 95:5 for HelMS(0.1)SD-30 and HelMS(0.1)SD-60 materials, respectively. On the other hand, the two-step postgrafting material, HelMS(0.1)GR, reached a lower anti:syn ratio of 86:14. A likely explanation resides in the mobility of the chiral moiety. In the grafting methodology it is more favored, since the chiral dopant is anchored onto the surface by two or three specific points, allowing the chiral molecule flip on bond and change its position and, therefore, reducing its stereoefficacy, as the attack of the different reagents is carried out by different positions, promoting a less diastereomeric efficacy. Contrastly, in the onepot approach (PMOs) the chiral ligand takes part of the helical wall, thereby providing more rigidity to the chiral auxiliary. The more rigid the chiral backbone, the more favored is the enantioselectivity and diastereoselectivity, since steric constrictions and geometrical orientation are introduced. These impediments could lead to the reagents to adopt a single orientation. Hence, this extra rigidity in combination with the steric impediments supplied by the helical geometry, previously stated, forces the reagents to adopt a specific orientation, resulting in an outstanding diastereomeric induction toward the ANTI diastereoisomer.39,40

of chirally dopant active species into the structure of the twisted rod channels. These new materials were compared in terms of enantioactivity with the references, conventional MCM-41 and helical rod-like MCM-41 materials. The Mannich asymmetric synthesis was selected as a model reaction. In this case, benzaldehyde reacts with a primary amine, aniline, and cyclohexanone to finally afford a β-amino ketone (Scheme 1). The catalytic results corresponding to the references and helical chirally doped PMO materials are collected in Figure 6.

Figure 6. Mannich reaction product distribution toward anti and syn diastereoisomers.

The MCM-41 SiO2 material afforded a mixture of two diastereoisomers in almost equal amount (racemic mixture) due to the linear channels structure. Surprisingly, the helical HelMS(0.1) reference addressed the reaction toward the formation of the preferred ANTI β-amino ketone, obtaining a 78:22 ANTI:SYN ratio. This result may be explained by the steric constrictions inflicted by the helical channels, which provide specific microenvironments that lead to the reagents to adopt a specific orientation favoring the formation of the ANTI instead the SYN enantiomer. A probable reason could be based on the right-handed to left-handed helices ratio. It was previously reported that the ratio of right-handed to lefthanded helices of mesoporous silica fibers was tunable by changing stirring speed and direction and the synthesis media basic concentration.37,38 In order to study the effect of the chirally dopant moiety, the reference MCM-41 was modified through grafting of the chiral tartramide covalently bonded to the silica silanol species. The material, named MCM-41-GR, afforded a significant higher enantioactivity toward the ANTI β-amino ketone compared to raw MCM-41 material (anti:syn ratio 76:24 vs 55:45, respectively). This result confirms the positive effect of the organic chiral tartramide moieties on the reaction detached from the helical effect. Taking into account the previous results, it follows that both reference helical HelMS(0.1) and chiral grafted MCM-41 GR materials achieved rather similar results (anti:syn ratio 78:22 vs 76:24, respectively). Finally, materials combining both features, helical and external chiral dopant

4. CONCLUSIONS The preparation of helical and supplementary chirally doped heterogeneous PMO materials was successfully accomplished by one-pot direct synthesis methodology. This approach was based on the use of dual achiral surfactants templating (weight ratio CTAB/PFOA = 0.1) and the addition of chiral dopant moieties and inorganic silica sources (molar ratio TEOS/bissilylated chiral precursor = 85:15) in the synthesis media. The obtained materials exhibited high specific surfaces, pore size distributions, and total pore volume in the range of mesopores. The mesoscopic order and the helical pitch of the final materials depend on the prehydrolysis time of the silica source. Additionally, the solid materials were, qualitatively and quantitatively, characterized by means of solid-state 29Si and 13 C NMR, which allowed a full depiction of the modified silica surface, corroborating the incorporation of the external chiral ligand into the inorganic helical rod mesoporous channels. The chiral PMOs materials led to a greater control of surface properties and presented a larger incorporation of organic species compared with the conventional grafting methodology. In terms of enantiocatalytic activity, the effect of the helical channel was found to have a positive influence on the 888

dx.doi.org/10.1021/la403728a | Langmuir 2014, 30, 881−890

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diastereoselectivity of the Mannich asymmetric synthesis, used as catalytic test. Likewise, the individual effect of the external chiral dopant was demonstrated to be also positive in the induced diastereoselectivity toward the desired product. Taking into account the previous results, the contributions of both combined effects were simultaneously studied and proved to be successfully synergetic toward the formation of the preferred diastereoisomer. The best results were achieved for the PMO materials synthesized through the one-pot approach, in which helical rod channels and chirally doped ligand contributed to the outstanding achievement of 95:5 diastereomeric ratios. The combination of both sterical restrictions favored the introduction of geometrical orientations that promote the reagents to adopt a specific orientation, resulting in the acquisition of impressive diastereomeric ratios.

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of reference mesoporous material MCM-41 and chiral bis-organosilane, list of the heterogeneous samples prepared in this work, values of helical/no helical fibers from SEM images and chiral PMOs 13C NMR solid state. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone +34 91 488 70 86, e-mail [email protected] (R.A.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Spanish government (CTQ200805909 and CTQ 2011/22707) is gratefully acknowledged. REFERENCES

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