Article pubs.acs.org/JPCC
(1R,2S)‑Ephedrine: A New Self-Assembling Chiral Template for the Synthesis of Aluminophosphate Frameworks Teresa Á lvaro-Muñoz,† Fernando López-Arbeloa,‡ Joaquín Pérez-Pariente,† and Luis Gómez-Hortigüela*,† †
Instituto de Catálisis y Petroleoquímica, ICP-CSIC, C/Marie Curie 2, 28049 Madrid, Spain Departamento de Química Física, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain
‡
S Supporting Information *
ABSTRACT: (1R,2S)-(−)-Ephedrine is used as a new structure-directing agent for the synthesis of nanoporous aluminophosphates. This molecule is selected based on the selfaggregating behavior through π−π type interactions between the aromatic rings and the presence of H-bond-forming groups. Additionally, this molecule possesses two chiral centers, which could enhance a potential transfer of chirality to the inorganic framework. Synthesis results showed that (1R,2S)-(−)-ephedrine is very efficient in directing the crystallization of the AFI-type structure in the presence of several catalytically active dopants. A combination of fluorescence spectroscopy and molecular mechanics simulations shows that ephedrine displays a great trend to self-assemble in water solution, establishing not only π−π type interactions between the aromatic rings but also intermolecular H-bonds between NH2 and OH moieties which compete with the formation of H-bonds with water. These molecules are invariably incorporated as aggregates within the AFI structure, regardless of the dopant introduced, showing a very strong trend to self-assemble within nanoporous frameworks as well. The stability of this supramolecular arrangement within the framework is due to a molecular recognition phenomenon based on the establishment of two H-bonds between the H atoms of the amino group and the O atoms of the hydroxyl group of the consecutive dimer, leading to an infinite supramolecular π−π H-bonded chainlike arrangement within the AFI channels. phosphate family of zeotypes (AlPO4-n series),9−13 in which the network is composed of AlO4 and PO4 tetrahedral units arranged in a strict alternation, also almost invariably require the use of organic molecules in their synthesis. However, with these AlPO frameworks, amines are used more often than quaternary compounds. Synthesis of new large-pore zeolite-like structures is an ultimate goal in zeolite science, particularly interesting for catalysis for these structures will allow the processing of large organic molecules. In searching for novel large-pore nanoporous structures, increasingly larger, bulkier, and more complex SDAs have been extensively used, leading to the discovery of a number of new zeolitic topologies.14 Using this approach, the first zeolitic materials containing 14-ring channels were first reported in 1996; UTD-1 (DON)15−17 was synthesized using a permethylated bis-cyclopentadienyl “sandwich” complex of cobalt, and similarly CIT-5 (CFI structure type)18,19 was prepared using a polycyclic amine. Recently, the first mesoporous chiral zeolite (ITQ-37) has been obtained by using a very large, rigid, and complex organic SDA.20 However, such intensification of the size and complexity of organic species is severely restricted by the chemical requirements of the organic molecules to be efficient SDAs. On the other hand, the thermodynamically disfavored crystallization of very open
1. INTRODUCTION Nowadays nanoporous materials, in particular zeolites, are finding new applications in materials science since their nanoscopic and crystalline structure and the associated properties allow their use to recognize and discriminate molecules with precisions that can be fundamental in all fields of molecular recognition phenomena. In particular, the design of solid sorbents and heterogeneous catalysts from nanoporous materials which combine the shape selectivity characteristic of these materials and enantioselectivity represents one of the biggest quests in zeolite science. Since the pioneering work of Barrer and Denny in the 1960s,1 organic compounds, particularly quaternary ammonium salts, have been extensively used in the hydrothermal synthesis of zeolite materials, which led to the discovery of a number of new framework structures and compositions.2 These organic molecules are often referred to as structure-directing agents (SDAs), since they direct the crystallization pathway toward a particular framework that would not be formed in its absence.3−7 The SDAs are encapsulated in the nanoporous structure during its crystallization, developing strong nonbonding interactions with the framework and thus contributing to the final stability of the system. For an organic molecule to be an efficient SDA, it has to fulfill a series of chemical requirements, like to have a moderate hydrophobicity, high solubility in the synthesis media, moderate rigidity, and high hydrothermal stability, and should develop strong nonbonding interactions with the nanoporous framework.8 The alumino© 2014 American Chemical Society
Received: November 12, 2013 Revised: January 24, 2014 Published: January 27, 2014 3069
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2. COMPUTATIONAL AND EXPERIMENTAL METHODOLOGIES A. Computational Simulations. Molecular-mechanicsbased simulations were performed to analyze the aggregation behavior of ephedrine (EPH) in aqueous solution in order to understand the molecular features governing the supramolecular chemistry that drives the formation of self-assembled aggregates in water. Molecular structures of ephedrine and water molecules were described with the cvff force field.28 Due to the strong basicity of ephedrine (pKa = 9.6)29 and the low pH of the synthesis medium (5−6, see Tables S1, S2, and S3 in the Supporting Information), protonated EPH (EPH+) molecules were studied. The atomic charge distribution of EPH+ was obtained from DFT calculations, using the B3LYP hybrid functional and the ESP charge calculation method, setting the total net charge to +1. The positive charge of the EPH+ molecules was compensated by including an equal number of Cl− anions in the simulations. The atomic charges in water molecules were −0.82 and +0.41 for oxygen and hydrogen, respectively.30 Due to the presence of four rotatable bonds, and the importance of molecular flexibility when studying struturedirecting and supramolecular aggregation issues,8,31 as well as in molecular recognition phenomena, an initial conformational analysis was performed. The conformational space of DPGH+ was scanned by means of the Conformers Calculation Module in Materials Studio,32 using a systematic grid scan search method and optimizing the molecular structures for each set of dihedral angles. The aggregation behavior of EPH+ molecules in water was studied by means of molecular dynamics (MD) simulations, under periodic boundary conditions (PBC), using the Discovery code as implemented in Material Studio.33 Sixteen EPH+ molecules and 16 Cl− anions were included in the simulation cell together with 640 water molecules. An initial equilibration period has been allowed, consisting of 100 ps of MD simulations in the NPT ensemble at 25 °C. The density of the systems along this initial MD simulation was averaged, and a frame in the last steps of the MD trajectory with a density close to the averaged value was selected as the starting configuration for the subsequent NVT study. MD simulations (750 ps) were run, keeping the temperature constant at 298 K. Of this simulation time, the first 250 ps were assumed as the equilibration period, and only the last 500 ps of the MD simulations were used for production. The aggregation behavior of EPH+ molecules was studied by analyzing the radial distribution functions (RDFs) of different sets of atoms [gαβ(r)]. B. Hydrothermal Synthesis of AFI materials. Nanoporous aluminophosphates were synthesized by hydrothermal methods using (1R,2S)-ephedrine (Sigma Adrich, 98%) as SDA. Gel molar compositions and synthesis conditions were systematically varied, as detailed in Tables S1, S2, and S3 in the Supporting Information. The incorporation of several catalytically active metals, such us magnesium, silicon, cobalt, or zinc, has been studied. Pseudoboehmite (Pural SB-1 77.5% Al2O3, Sasol) and phosphoric acid (Sigma-Aldrich, 85%) were used as sources of Al and P, respectively. A wide scan of the experimental conditions (organic and water contents, presence of dopants, temperature, and crystallization times) was performed in order to obtain pure phases.
frameworks, which are intrinsically less stable than denser frameworks, has to be compensated for by the establishment of strong nonbonding interactions with guest, SDAs in this case, species. Based on this, in a quest for new large and efficient organic SDAs, new concepts in the design of the organic molecules are to emerge,21 especially involving the two main aspects of the SDA chemistry: (i) the increase of the SDA molecular size while fulfilling the chemical requirements of the SDAs and (ii) the enhancement of the efficiency of the SDA molecules by maximizing their nonbonding interactions with the frameworks. As previously mentioned, the increase of the SDA molecular size is currently reaching a limit; this is so because their choice as SDAs has almost invariably considered the features of single molecular units. Supramolecular chemistry has only rarely been applied in structure direction, in sharp contrast to the wide and very successful use of supramolecular micelle arrangements of surfactants in the synthesis of mesoporous materials. In this context, a new concept in structure direction has been recently developed by us22−26 and by Corma et al.;27 the strategy consists of the use of molecules that self-assemble as supramolecular aggregates as SDAs, with the aggregates being the actual structure-directing entities. This concept permits the use of relatively simple molecules with suitable size, hydrophobicity, and basicity properties, to create more complex and larger pore zeolite topologies due to their aggregation in supramolecular arrangements. One of the most common driving forces for the self-assembly of organic molecules in aqueous solution is the presence of aromatic rings that tend to aggregate due to the establishment of π−π type interactions, which has been the main strategy we have followed to date. On the other hand, the most common driving force for molecular recognition phenomena is the development of intermolecular H-bonds between particular donor/acceptor groups of different molecules. However, as previously mentioned, quaternary ammonium compounds, where no H atoms of amino groups susceptible of developing H-bonds are present, have been more often used in the synthesis of zeolites. In contrast, amines with potentially Hbond-forming H atoms are more efficient for the synthesis of nanoporous aluminophosphate frameworks. The possibility of forming H-bonds would in principle enable a supramolecular ordering through molecular recognition phenomena among the SDA molecules occluded within nanoporous frameworks. Based upon these grounds, (1R,2S)-(−)-ephedrine (EPH) has been used as SDA for the synthesis of hydrophilic aluminophosphate frameworks. This molecule has been rationally selected based on the presence of aromatic rings which tend to self-assemble through π−π type interactions, HN and OH groups susceptible of developing intermolecular Hbonds, and high conformational flexibility due to the presence of four rotatable bonds, which would facilitate the formation of those H-bonds. The synergy of these particular molecular features in EPH will drive the molecule to develop a very strong self-assembling trend to form supramolecular aggregates in aqueous solution that will act as structure-directing species for the crystallization of nanoporous aluminophosphate frameworks. Although this will not be particularly analyzed in this work, this molecule has also the advantage of having two asymmetric C atoms, imparting a strongly asymmetric nature to the molecule. 3070
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In a typical synthesis, pseudoboehmite and the corresponding metal source (Mg(CH3COO)2·4H2O (Sigma-Aldrich, 99.5%), Co(CH3COO)2·4H2O (Sigma Aldrich 98%), Zn(CH3COO)2·2H2O (Sigma-Aldrich, 99%), or tetraethylorthosilicate, TEOS (Merck, 98%)), were added to a phosphoric acid solution, and the mixture was stirred for 1 h. (1R,2S)-Ephedrine was then added to this mixture and was stirred for about 2 h to obtain a uniform gel. The gels were then transferred into Teflon-lined stainless steel autoclaves with a capacity of 14 cm3, which where heated statically at the required temperature (130−180 °C) under autogenous pressure for a specific period of time (from 3 to 120 h). The resulting solids were collected by filtration, washed thoroughly with water and ethanol, and dried at room temperature. Aqueous solutions of (1R,2S)-ephedrine were prepared by adding equimolar amounts of the amine and HCl. In this way, 0.1, 0.05, 0.01, 0.001, and 0.0001 M aqueous solutions were obtained and studied by fluorescence spectroscopy. C. Characterization. The obtained solids were characterized by different physicochemical techniques. Powder X-ray diffraction (XRD) patterns of samples were recorded on a Philips X́ PERT diffractometer using CuKα radiation with a Ni filter. The crystal morphology was studied by scanning electron microscopy (SEM) using a Hitachi TM-1000 Tabletop microscope. The organic content of the samples was studied by thermogravimetric analysis (TGA) using a Perkin-Elmer TGA7 instrument. Chemical analyses were obtained by ICPAES (Fluxy-30, Claisse). UV−visible diffuse reflectance spectra were registered on a Cary 5000 Varian spectrophotometer equipped with an integrating sphere using the synthetic polymer Spectralen as reference. MAS NMR spectra were recorded with a Bruker AV 400 WB spectrometer, using a BL7 probe for 13C and a BL4 probe for 31 P. 1H to 13C cross-polarization spectra were recorded using π/2 rad pulses of 4.5 μs for 1H, a contact time of 5 ms, and a recycle delay of 3 s. For the acquisition of the 13C spectra, the samples were spun at the magic angle (MAS) at a rate of 5−5.5 kHz. For 31P, π/2 rad pulses of 4.25 μs and recycle delays of 80 s were used; these spectra were recorded while spinning the samples at ca. 11 kHz. 29Si CP MAS NMR was recorded with a 4 mm probe spinning at 10 kHz. A π/2 pulse of 3 μs, contact time of 6 ms, and recycle delay of 5 s were used. The aggregation state of the molecules in solution and in the solid samples was studied by fluorescence spectroscopy. Liquid and solid state UV−visible fluorescence emission spectra were recorded in a RF-5300 Shimadzu fluorimeter. The fluorescence spectra were registered in the front-face configuration by a solid sample holder in which the samples were oriented 30° and 60° with respect to the excitation and emission beams, respectively. Liquid solutions of the SDA samples were placed in 1-, 0.1-, or 0.01-mm pathway quartz cells, depending on the concentration of the EPH solution, whereas the fluorescence spectra of the solid samples were recorded by means of thin films supported on glass slides ellaborated by solvent evaporation from a dichloromethane suspension of the solid AFI samples.
Figure 1. Fluorescence spectra of ephedrine·HCl of increasing concentrations (M) in aqueous solution. Spectra of solutions with 0.1 and 0.05 M concentrations have been smoothed because of the low signal-to-noise ratio due to the low optical pathway of the used cell.
emission from the aromatic system of ephedrine monomers, since at this low concentration the presence of EPH monomers should be predominant. A progressive increase of the concentration results in the appearance of a new broad band at longer wavelengths (between 325 and 425 nm, centered at 360 nm), which becomes the predominant band as the concentration rises to 0.1 M (pink line). The occurrence of this new band with the increase in the concentration of the molecules led us to assign it to the fluorescent emission from EPH molecules in an aggregated state; π−π type interactions in the aggregated state cause a stabilization of the electronic levels leading to the observed bathochromic shift in the emission band. Worth noting is the different fluorescence behavior observed for ephedrine compared to other self-assembling aromatic amines previously studied by us (benzylpyrrolidine, BP, and (S)-N-benzyl-2-pyrrolidinemethanol, BPM).23,25 A much larger shift toward higher wavelengths is observed for ephedrine molecules upon an increase in the concentration (BP and BPM shift occurred from 282 nm, monomers, to 322 nm, aggregates). This shows that the interaction between the EPH aromatic rings in aggregates is much stronger than that for BP and BPM. On the other hand, supramolecular aggregation for the latter molecules was only predominant at a concentration of 1 M; in contrast, EPH aggregates are already observed at concentrations as low as 10−3 M (Figure 1, blue line) and are the predominant species at a concentration of 0.1 M (pink line). Both observations suggest a much stronger supramolecular self-assembly behavior for EPH molecules. Molecular Dynamics Simulations. We then analyzed the EPH behavior in aqueous solution by MD simulations to characterize the configuration of the supramolecular aggregates. Due to the conformational flexibility of EPH, and the importance in structure-direction and molecular-recognition phenomena, we first performed a conformational analysis (Figure 2). Out of the 140 conformational configurations analyzed, only seven different conformers were found to be stable. Figure 2 shows the most stable ones, where intramolecular H-bonds are established; the other three conformers,
3. RESULTS A. Aggregation of EPH+·Cl− in Aqueous Solution. Fluorescence Spectroscopy. Figure 1 displays the fluorescence spectra of aqueous solutions of ephedrine hydrochloride (EPH+Cl−) at increasing concentrations. At the lowest concentration studied (10−4 M, black line), a main fluorescence band is observed centered at 282 nm, which is assigned to the 3071
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0.361 for hn2). This shows that the higher stability of such Hbond is due to a more favored spatial configuration. We next studied the behavior of EPH in aqueous solution at a concentration similar to that in the aqueous synthesis gels (Figure 3). Initially a conformational analysis in water was performed by analyzing intramolecular RDFs (Figure 3A). We observed a very intense signal indicative of the formation of Hbonds between the O atom of the hydroxyl group and the hn1 atom of the amino group (red line), much stronger than that corresponding to hn2 (green line), showing that such intramolecular H-bonds are maintained during all the simulation in most of the EPH+ molecules. Moreover, a very prominent signal at 3.0 Å for the c3−h distance is observed (blue line). These results clearly indicate that the most stable conformer in vacuum, conformer 1 (Figure 2), is also the predominant one in aqueous solution, evidencing a notable rigidity of the molecule despite the presence of four rotatable bonds and of water molecules that would compete for the formation of H-bonds with the polar groups of EPH. These intramolecular H-bonds must enhance the asymmetric nature of EPH molecules, reducing their conformational flexibility. Conformational flexibility can reduce the degree of asymmetry during crystallization of host−guest materials. Supramolecular aggregation was then analyzed; Figure 3B shows the intermolecular RDF between aromatic C atoms (cp−cp, black line). The high-intensity peak between 4 and 6 Å indicates a preferential location of the aromatic C atoms at such distances, showing a strong π−π stacking, and hence a strong
Figure 2. Most stable conformers of EPH+, with corresponding relative energy (in kcal/mol); H-bonds are shown as dashed blue lines. Other less stable conformers without intramolecular H-bonds have been omitted. Distinguishable C(CH3)−O(OH) distances are highlighted with yellow dashed arrows.
where no H-bonds are developed, are much less stable (relative energies of 9−17 kcal/mol with respect to conformer 1). Of the four H-bonded conformations, three of them (1, 2, and 3) develop intramolecular H-bonds with one of the H atoms of the amino group (hereafter called hn1), while conformer 4 develops the intramolecular H-bond with the other H (hn2). Looking at the relative energy differences, it becomes clear that the most stable intramolecular H-bond is developed with hn1, which in fact has a slightly smaller charge (0.324 e for hn1 vs
Figure 3. Intramolecular (A) and intermolecular (B and C) radial distribution functions (RDFs) of different sets of atoms during the MD simulations of EPH+Cl− in aqueous solution (w indicates water molecules). (A) includes the molecular structure and atom types of EPH+. (D) Snapshot (t = 375 ps) of the arrangement of EPH+ aggregates, showing the formation of intermolecular H-bonds (dashed blue lines) and the π−π type interactions (dashed yellow lines) (water molecules have been omitted for clarity). 3072
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CoAPO-5, SAPO-5, and ZnAPO-5 pure materials, which were further characterized. SEM (Figure S2 in the Supporting Information) showed two different kinds of crystalline morphologies: SAPO-5 crystallizes as large hexagonal prisms, with a crystal length close to 50 μm; in contrast, MgAPO-5, CoAPO-5, and ZnAPO-5 are obtained as aggregates of long needles. The incorporation of organic molecules was studied by thermogravimetric analysis and 13CP MAS NMR. The TGA profiles of the samples studied (Figure S3 in the Supporting Information) show four different weight loss steps. The first weight loss, at temperatures below 200 °C, can be attributed to water desorption. The other weight losses are due to the decomposition of the template, which occurs in several steps. The chemical integrity of EPH inside the framework was verified by 13CP MAS NMR (Figure 4), showing that the molecule is incorporated intact within the structure in the different materials (see assignments of the different bands in Figure 4).
supramolecular aggregation through the aromatic rings, between EPH+ molecules, in agreement with the fluorescence results. In consequence, no interaction between the aromatic H atoms with water molecules is observed (hcp−ow, gray line). A strong peak at 1.7 Å between the H atom of the hydroxyl group of EPH+ and O atoms of water is observed (ho−ow, blue line), much stronger than that corresponding between the O atom of EPH+ and H atoms of water (oh−hw, red line), indicating the formation of H-bonds between the H-atom of the EPH+ hydroxyl group and the water O atom. H-bonds between H atoms of EPH+ amino group and water O atom are also observed, which are stronger with hn2 (dark cyan line) than with hn1 (green line) (see Figure 3A for nomenclature). In contrast, no interaction of the amino N atom with water H atoms is appreciated (n−hw, orange line). We then analyzed the intermolecular interaction between the polar groups of EPH+ (Figure 3C). No H-bond intermolecular interaction was observed between the amino groups (black line) of different EPH+ molecules nor between the hydroxyl groups (blue). In contrast, a clear H-bond intermolecular interaction at a distance of 1.8 Å is observed between the H atoms of the amino group and the O of the hydroxyl group (red and green lines, hn1−oh and hn2−oh, respectively). At the same time, as previously observed, one of the H atoms of the amino group (hn1) is involved in an intramolecular H-bond with the O atom of the hydroxyl group within the same molecule (Figure 3A, red line); such intramolecular H-bond is mostly developed with hn1 (rather than with hn2). Figure 3D shows a snapshot (at time = 375 ps) of the type of aggregates formed, where two self-assembling driving forces are present, π−π type interactions between the aromatic rings (dashed yellow lines) and intra- and intermolecular H-bonds between the H atom of the amino group and the O atom of the hydroxyl group. On the other hand, H atoms of the hydroxyl group (ho) and of the hn2 amino group interact preferentially with O atoms of water (Figure 3B). Therefore, the type of supramolecular aggregates found here suggests a molecular recognition phenomenon of EPH molecules in water, both at an intramolecular level (through the H-bonds between O and H−N, determining the conformation) and at an intermolecular level (through π−π interactions on the one side and H-bonds between O and H−N atoms, determining the type of aggregate). B. Hydrothermal Synthesis of AFI Materials. The use of (1R,2S)-ephedrine as SDA under different gel compositions and hydrothermal conditions has enabled the crystallization of AFI-type materials (Figure S1 and Tables S1, S2, and S3 in the Supporting Information). The AFI-type structure is composed by one-dimensional noninterconnected 12-membered ring cylindrical channels. The synthesis results indicate that (1R,2S)-ephedrine is only able to direct the synthesis toward AFI-type materials in the presence of dopants. It was not possible to synthesize undoped AlPO-5 since in the absence of dopants a low-dimensional unknown framework with a high organic content (which we refer to as phase X), AEN (as a result of EPH degradation, at high temperatures and long crystallization times), or a trydimite-like dense AlPO framework crystallize (see Table S1). However, the incorporation of dopants such as Co2+, Zn2+, Mg2+, or Si4+ drives the crystallization at high temperatures toward AFI-type materials (see Tables S2 and S3 in the Supporting Information). In general, compositions of 0.925 Al2O3:0.15 MeO:1 P2O5:2 EPH:100 H2O allowed for the crystallization of MgAPO-5,
Figure 4. 13C CP MANMR of the different AFI materials studied and assignment of the different signals.
The incorporation of dopants into the AFI structure was verified by different techniques. 31P MAS NMR demonstrated the incorporation of Mg and Zn within the AFI framework (Figure S4 in the Supporting Information). The spectra of these samples consists of a peak at −30 ppm, which is characteristic of tetrahedral P atoms in a P(4Al) configuration in the AFI network.34 In addition, two peaks at −24 and −22 ppm are observed in MgAPO-5 and ZnAPO-5, respectively, due to the presence of P (1Mg, 3Al)35 and P (1Zn, 3Al)36 environments. The incorporation of Si was evidenced by 29Si MAS NMR. The spectrum of SAPO-5 (Figure S5 in the Supporting Information) consists of a broad band between −85 and −105 ppm, which involves the different Si environments (Si−(OAl)n(OSi)4−n. These multiple Si environments occur as a result of the combination of the single P by Si substitution mechanism with the simultaneous substitution of a pair of neighboring Al and P atoms by two Si atoms. Finally, the incorporation of Co in the framework of the CoAPO-5 sample was evidenced by UV diffuse reflectance spectroscopy by the presence of three bands between 500 and 700 nm (Figure S6 in the Supporting Information), characteristic of the 4A2(F) → 4 T1(P) transition of tetrahedral Co(II).37 The dopant content, 3073
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introduced in the framework, a broad band between 300 and 500 nm is observed, which corresponds to the presence of SDA aggregates occluded inside the AFI pores.23,24 Remarkably, and in contrast with previous results with benzylpyrrolidine and (S)-1-benzyl-2-pyrrolidinemethanol,23 we observed that EPH is always incorporated as aggregates within the AFI structure, showing a very strong trend to self-assemble also within nanoporous frameworks. Different maxima in the aggregates emission range can be observed as a function of the dopant introduced. Due to the confinement effect of the AFI onedimensional channels, we tentatively assign such different bands either to distinct interactions of the molecules with the dopants incorporated or to slightly different π−π stackings in the aggregates (different distances/orientations between the aromatic C atoms).
calculated by ICP, was 1.1 Mg/u.c., 0.8 Si/u.c., 1.0 Zn/u.c., and 1.3 Co/u.c. for MgAPO-5, SAPO-5, ZnAPO-5, and CoAPO-5, respectively. C. Aggregation of EPH within the AFI Materials. After characterizing the aggregation state of EPH in solution, we studied their aggregation state inside the AFI framework. Solid state UV−visible fluorescence spectroscopy results are presented in Figure 5. Fluorescence results evidence the
4. DISCUSSION In this work, (1R,2S)-ephedrine, an alkaloid derivative found in various plants and commercially available, has been shown as a new chiral organic SDA with a great trend to self-assemble when directing the crystallization of nanoporous aluminophosphates. A combination of fluorescence spectroscopy and molecular simulations shows a great potential of this molecule to self-aggregate in aqueous solution through the aromatic rings by π−π type interactions. Such interactions are much stronger for EPH than for our previously studied aromatic amines, benzylpyrrolidine (BP) and (S)-N-benzyl-2-pyrrolidinemethanol (BPM),23 as evidenced by the longer red-shift of EPH upon an increase in the concentration, and consequently aggregation, of the molecules. This implies a stronger trend of EPH to selfassemble in water, as confirmed by the predominance of aggregates at a concentration of 0.1 M, at which BP and BPM
Figure 5. Solid state fluorescence spectra of the different AFI materials obtained with EPH.
complete absence of monomers in all the materials (there is no emission band at around 280 nm). Regardless of the dopant
Figure 6. Molecular structure (A) and intermolecular RDFs between aromatic C atoms (B) and between H-bonded-to-N and O atoms of different molecules related to EPH (C). 3074
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Figure 7. Geometry-optimized molecular structure of BPM (A) and EPH (C) aggregates within the AFI channels and intermolecular RDFs between H-bonded-to-N atoms (B, hn1 for EPH) and between H-bonded-to-N and O atoms (D, hn1 for EPH). C, H, N, and O atoms are displayed in gray, white, blue, and red, respectively.
reduce the π−π self-aggregation. Finally, the lowest π−π selfassembly is observed for the EPH derivative with one less C atom between the aromatic ring and the N atom (ephed(n-1), blue line). In this case, this weaker π−π self-aggregation is due to the shorter separation between the aromatic (hydrophobic) and the polar (amino and hydroxyl groups) sides of the molecule, which does not enable an effective interaction of each side with the corresponding species of similar chemical nature (other aromatic rings for the former and water or polar sides of other molecules for the latter). In summary, the trend to π−π self-assemble of these molecules follows the order ephed ∼ ephed(noOH) > Me-ephed ∼ ephed(cyc5) > ephed(nh3) > ephed(n-1). Next we analyzed self-assembly through the formation of intermolecular H-bonds between the amino and hydroxyl groups (Figure 6C; ephed(noOH) was not studied due to the absence of O atoms). Surprisingly, we observed that EPH is the only molecule where effective intermolecular H(N)···O Hbonds are formed in aqueous solution. Any alteration of the EPH molecular structure involves the disappearance of this type of H-bond-driven self-assembly. Introduction of bulky substituents on N, in Me-ephed and ephed-cyc5 (red and purple lines, respectively), prevents the formation of such Hbonds by steric reasons. Reduction of the chain length in ephed(n-1) (blue line) probably impedes an effective separation and orientation of the polar and apolar molecular sides. Finally, removal of the methyl substituent (ephed(nh3), green line) possibly brings more strong H-bond interactions with water molecules, thus competing with the formation of Hbonds with adjacent organic molecules. In summary, our results demonstrate that EPH has a very particular molecular structure which imparts a very strong trend to self-assemble through π−π
are predominantly still forming monomers; it is only at 1 M concentration that these molecules form mostly aggregates. In an attempt to unravel the molecular features governing such strong trend to self-assemble of EPH, MD simulations of related molecules were performed (Figure 6), where the effects of the amino substitution (Me-ephed and ephed(nh3)), of the separation between the aromatic and the amino moieties (ephed(n-1)), of the presence of the hydroxyl group (ephed(noOH)), and of having bulky and rigid N-substituents (ephed(cyc5)) (see Figure 6A) were analyzed. We first considered the effect on the π−π-driven self-assembly through the aromatic rings (by analyzing the RDFs between aromatic C atoms and comparing with that of ephedrine, black line in Figure 6B). As expected, the presence of the hydroxyl group does not alter the π−π-driven self-aggregation (ephed(noOH), dark yellow line); ephed (black line) and ephed(noOH) display the highest π−π aggregation. The increase of the steric volume on the polar side of the molecule by adding another methyl group (Me-ephed, red line) or a five-membered ring (ephed(cyc5), purple line) reduces the π−π self-aggregation, possibly because of a steric repulsion generated by the presence of these bulky groups. Removal of the methyl substituent in EPH (ephed(nh3), green line) involves a further reduction of the π−π self-aggregation. In this case, analysis of the different RDFs suggests that this lower π−π self-assembly is caused by a higher interaction of the unsubstituted (and more polar) R−NH3 amino group with O atoms of water molecules, as evidenced by the higher intensity of the RDF between the H atom of the amino group and water O atom in both the first (at 1.9 Å) and especially the second (at 3.3 Å) shells (see Figure S7 in the Supporting Information). Such stronger hydrophilic interaction and stronger water coordination shell might compete with and 3075
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assembly and molecular recognition. A combination of UV−vis fluorescence spectroscopy and molecular simulations has shown that this molecule displays a very strong trend to selfassemble in aqueous solution through two types of intermolecular interactions, through π−π type interactions between aromatic rings, leading to a π−π stacking, and through H-bonds between H-atoms of amino groups and O atoms of hydroxyl groups of adjacent molecules. This leads to large selfassembled supramolecular aggregates. Interestingly, such double self-assembly behavior is reproduced when ephedrine molecules are occluded within the onedimensional channels of the AFI framework. Fluorescence results show that only aggregates are incorporated within the channels. The electrostatic repulsion generated by the close location of positively charged N atoms of protonated ephedrine molecules in adjacent aggregates is compensated for by the development of two H-bonds between the H atoms of amino groups and O atoms of hydroxyl groups of consecutive aggregates. Indeed, our results show that (1R,2S)-(−)-ephedrine has a very particular molecular structure that enables such double self-assembling nature through aromatic rings and intermolecular H-bonds, which is canceled upon any slight modification of the molecular structure. This leads to supramolecular infinite ephedrine chains occluded within the AFI channels, stabilized by π−π type interactions between aromatic rings on the one side and H-bonds between amino and hydroxyl groups on the other.
type interactions on the aromatic side and through H-bond interactions on the polar molecular side, providing potentially an intense molecular-recognition capacity. We finally studied the correlation of this particular selfassembly behavior of EPH in water with its incorporation within the one-dimensional channels of the AFI structure and compared it with the self-assembling molecule we previously studied, (S)-N-benzyl-2-pyrrolidine-methanol (BPM) (Figure 7). Previous results showed a much weaker supramolecular aggregation of BPM within the AFI channels;23,24 this was at least partially due to the electrostatic repulsion generated by the close location of the positively charged H amino atoms in consecutive dimers (Figure 7A) (indeed, loading of neutral BPM molecules showed a much stronger aggregation).26 This close location of the positive charges is evidenced by the intense peak of the RDF between the H atoms of the amino groups in consecutive aggregates (Figure 7B, black line). Indeed, a similar close location of the positively charged H atoms of the amino groups, and hence a similar electrostatic repulsion, occurs when consecutive EPH aggregates are packed within the AFI channels (Figure 7B, gray line) (a complete study about the location and supramolecular arrangement of EPH molecules within the AFI channels is out of the scope of the present work but will be provided in a forthcoming publication). However, our fluorescence results evidence the invariable incorporation of EPH molecules within the AFI channels as aggregates (Figure 5), even though a similar electrostatic repulsion would be expected. Therefore, it becomes clear that some other interaction must be present in order to compensate for such repulsion. Molecular simulations in aqueous solution showed a strong intermolecular H-bond interaction between the O atom of the hydroxyl group and the H atoms of the amino group of EPH molecules (Figure 3B). Interestingly, molecular simulations results show that these H-bond interactions are maintained within the AFI framework between H(hn1) and O(H) atoms (Figure 7C). Indeed, a very strong H-bond interaction at a distance of 2.0 Å is observed between these atoms in EPH (Figure 7D, gray line). In fact, two intermolecular H-bonds are developed between consecutive aggregates (Figure 7C, dashed blue lines). In contrast, similar H-bond interactions in BPM are much weaker (the intensity is much lower, and the H-bond distance is larger, 2.4 Å, Figure 7D, black line), and in this case, only one H-bond per two BPM aggregates is developed. Therefore, this strong trend of EPH to self-assemble through π−π type and double H-bond interactions is responsible for the strong supramolecular behavior observed when occluded within the nanoporous framework, which leads to the development of infinite supramolecular chains connected by π−π type and Hbond interactions within the one-dimensional channels of the AFI framework. Hence, our work shows a new SDA molecule, (1R,2S)-ephedrine, with a great potential of molecular recognition, and consequently to self-assemble, and hence a potential ability to direct the crystallization of large-pore frameworks. In addition, it has the advantage of possessing two asymmetric atoms, imparting an asymmetric character which is enhanced by the rigidity generated by the development of intramolecular H-bonds in water (Figure 3C).
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ASSOCIATED CONTENT
* Supporting Information S
Synthesis experiments, X-ray diffraction patterns, SEM images, thermogravimetric analyses, 31P and 29Si MAS NMR, UV− visible diffuse reflectance spectra, and additional radial distribution functions. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: +34-91-5854785. Fax: +34-91-5854760. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research leading to these results has received funding from the Spanish Ministry of Science and Innovation MICINN (projects MAT2009-13569 and MAT2012-31127) and the European Research Council, under the Marie Curie Career Integration Grant program (FP7-PEOPLE-2011-CIG), Grant Agreement PCIG09-GA-2011-291877. L.G.-H. acknowledges the Spanish Ministry of Education and Science for a Juan de la Cierva contract.
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REFERENCES
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5. CONCLUSIONS In this work (1R,2S)-(−)-ephedrine has been used as a new chiral structure-directing agent (SDA) for the synthesis of nanoporous aluminophosphates with a great potential of self3076
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