Selective Crystallization of Urea-Functionalized Capsules with

Growth Des. , 2009, 9 (4), pp 1985–1989. DOI: 10.1021/cg801299a. Publication Date (Web): February 5, 2009. Copyright © 2009 American Chemical Socie...
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Selective Crystallization of Urea-Functionalized Capsules with Tunable Anion-Binding Cavities Radu Custelcean* and Priscilla Remy Chemical Sciences DiVision, Oak Ridge National Laboratory, PO Box 2008, MS-6119 Oak Ridge, Tennessee 37831

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1985–1989

ReceiVed NoVember 28, 2008

ABSTRACT: Herein we report crystallization of self-assembled capsules functionalized with urea hydrogen-bonding groups as a means for selective separation of sulfate anion. The high complementarity and the rigid environment found in such crystalline systems impart strong discrimination between anions of different shape, like sulfate and sulfite, or anions of the same shape but slightly different size, like sulfate and selenate, with selectivity that exceeds that observed in sulfate-binding protein. Similar to natural receptors, these crystalline capsules completely isolate the anions from the aqueous solvent by encapsulating them inside rigid cavities lined with complementary hydrogen-bonding groups. Furthermore, the capsules are made from flexible building blocks, whose structure and relative orientation in the crystal can be allosterically regulated to fine-tune the anion selectivity. These characteristics suggest that crystallization of such urea-functionalized capsules from simple and flexible components represents a particularly promising approach for selective anion separation from highly competitive aqueous environments. Introduction Selective and efficient binding of substrates by natural or synthetic receptors generally requires a combination of high complementarity and organizational rigidity of the receptor′s binding site.1,2 The former insures strong guest binding by optimizing the strength and number of host-guest interactions, while the latter is required to prevent the host from adjusting its geometry to accommodate competing guests. Although these foundational concepts have long been recognized, their successful implementation in synthetic receptors remains extremely challenging, especially in highly competitive aqueous environments.3-5 For anion separation, maximum selectivity can in principle be achieved by completely sequestering the anion from the surrounding solvent via encapsulation with an array of strong, well-positioned, and geometrically constrained interactions.6 Proteins have a remarkable ability to bind their substrates with high affinities and specificities by sequestering them deep inside their structures, into well-defined and rigid biding cavities. Although inherently flexible, protein receptors are organizationally rigid, as they are structurally encoded to adopt only particular conformations that maximize the binding of a specific substrate and discriminate against competing substrates based on structural differences of less than 0.1 Å.7 Another characteristic ability of proteins is that the flexibility of their building blocks allows them to undergo conformational changes in response to allosteric ligand binding, thereby modulating their affinity and selectivity for substrates.8 Anion-binding proteins9-11 are exceptional receptors with unparalleled affinities and selectivities in water. In direct contrast, most synthetic anion receptors12 are far less effective than their natural counterparts, as they are neither complementary enough to the targeted guests nor sufficiently rigid. One major impediment in the development of highly selective artificial anion receptors is that the synthesis of rigid, internally functionalized hosts capable of fully encapsulating their guests is labor intensive and inefficient. As an alternative to molecular receptors, we and others have been exploring organic crystalline phases functionalized with hydrogen bonding groups for maximal three-dimensional com* Corresponding author. E-mail: [email protected]. Fax: (865) 574-4939.

plementarity to anions.6,13-17 Although these crystals do not act as hosts in the traditional sense, as the organic frameworks may need the anions to form in the first place, and typically cannot exist on their own, an important feature that makes crystalline materials attractive anion-binding platforms is their stiffer environment, which is expected to provide the organizational rigidity that has been so elusive in molecular hosts. Our approach involves in situ self-assembly of cationic anionbinding frameworks from simple organic building blocks, through competitive crystallization from aqueous anionic mixtures.13 This quick and efficient process thus replaces the laborious step-by-step synthesis typically involved in the assembly of traditional receptors. Introduction of molecular recognition elements by functionalization of the organic components allows for a control of selectivity based on anions′ size, shape, and specific interactions with the framework. Another advantage of crystalline frameworks is that the rationalization of the observed selectivities is facilitated by their X-ray structural analysis that provides direct information about the anion binding site, unlike the case of molecular receptors, whose actual binding geometry in solution may differ significantly from the pictures revealed by X-ray in their crystals. We have recently discovered that the simple urea-functionalized tripodal ligand L1 self-assembles from water in the presence of Mg2+ and various oxodianions like SO42-, SeO42-, SO32-, and CO32-, into crystalline capsules (C1) held together in a three-dimensional framework by Mg(H2O)62+ cations acting as hydrogen-bonding bridges (Figure 1, left).13f The capsules are internally functionalized by six urea groups18 providing 12 NH hydrogen-bond donors that are complementary to tetrahedral oxoanions such as sulfate and selenate. Despite the very flexible nature of L1, the water bridges hold the capsules rigidly in place, so that the anion-binding cavity retains its shape complementarity even in the presence of pyramidal sulfite or trigonal-planar carbonate anions. This ideal combination of high complementarity and rigidity resulted in unprecedented selectivity for the tetrahedral sulfate and selenate anions. The anion encapsulation by C1 is characteristic to the solid state, as only 1:1 sulfate/L1 complexes could be detected in solution. This epitomizes the advantage of using crystalline materials for anion recognition

10.1021/cg801299a CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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Figure 1. Sulfate encapsulation by the crystalline self-assembled capsules C1 and C2. Two molecules of L1, shown as stick models, are held together by six Mg(H2O)62+ (left) or six Li(H2O)+ (right; water protons not shown), depicted as ball and stick models. Sulfate is shown as a space-filling model.

and separation, as they offer unique anion binding modes that may be inaccessible in solution. The anion cavity in C1 was large enough to accommodate either SO42- or SeO42-, which resulted in poor selectivity (separation factor of 5) for sulfate against selenate. Given their similar size (rX-O ) 1.49 and 1.65 Å for X ) S and Se, respectively),19 and almost identical basicity, finding receptors capable of discriminating between these two anions represents a fundamental challenge in anion recognition and separation. Apart from sulfate-binding protein, which separates sulfate from selenate by a factor of 40,10 to the best of our knowledge, no synthetic host has demonstrated effective discrimination between these two very similar anions. Here we show that the binding site in C1 can be allosterically regulated by the external bridging cation, thereby fine-tuning the SO42-/SeO42- selectivity. Specifically, substituting Li(H2O)+ for Mg(H2O)62+ yields similar capsules C2 in the solid state, (Figure 1, right) but with substantially reduced cavity size. This allows for effective separation of sulfate from selenate by competitive crystallization, with an observed selectivity exceeding that of sulfate-binding protein. Results Crystallization of L1 in the presence of Li2SO4, from H2O/ MeOH solutions afforded truncated octahedral-shaped crystals of 1 with the composition Li2SO4(L1)2(H2O)2, as determined by single-crystal X-ray diffraction (Figure 2) and elemental analysis. The crystal structure of 1 consists of S6-symmetrical capsules (Figure 2a) containing 2 molecules of L1 held together by 6 H2O-Li+ units through Li · · · O(urea) coordination and water · · · pyridine hydrogen bonding (Figure 2a). Each H2O-Li+ pair sits on a S6 axis and is shared by three neighboring capsules (Figure 2b), with each Li+ assuming tetrahedral coordination geometry. This results in a 6,3-coordination framework (Figure 2c) belonging to the cubic Pa3j space group, where the SO4(L1)22- and Li(H2O)+ ions serve as octahedral and triangular nodes in a default pyrite (FeS2) net (same symmetry and topology).20 The C2 capsules are internally functionalized with six urea groups that encapsulate the sulfate by 12 hydrogen bonds13f,21,22 (Figure 2d), the optimal coordination number for this anion according to previous theoretical calculations.23 Although similar sulfate coordination was found in the related C1 capsules, there are some subtle but significant differences. First, C2 has higher symmetry (S6) compared with C1 (i), which results in a different sulfate binding mode in this case, with 3 urea groups each binding an O-S-O edge and the other 3 ureas each binding

Figure 2. Crystal structure of 1. (a) Coordination capsule with L1 shown as stick, H2O-Li+ as ball and stick (water protons not shown), and sulfate as space filling models. Water · · · pyridine hydrogen bonds are shown as dashed lines. The purple lines linking the Li+ cations are for guiding purpose only, to illustrate the octahedral shape of the capsule. b) Linking of the capsules (stick model) by H2O-Li+ (ball and stick model), via Li-O coordination and water-pyridine hydrogen bonding. (c) Coordination framework assembled from octahedral capsules (sticks) and triangular Li+ nodes (balls). (d) SO42- binding site consisting of 12 hydrogen bonds (dashed lines) from six urea groups. The sulfate is disordered over two positions (only one orientation shown) to emulate the S6 symmetry of the cage.

an O vertex of the tetrahedral anion, compared with 5 edgebinding and one vertex-binding ureas in C1. Second, the size of C2 is significantly smaller, with a distance between the tertiary N atoms of the two L1 ligands of 9.17 Å, which is 0.48 Å shorter than the corresponding distance in C1. This causes the sulfate to drop lower inside C2, departing from the cavity′s center by 0.27 Å, presumably to minimize the repulsive interaction between the lone pair of the tertiary N atom of L1 and the apical O atom of SO42-. These structural differences are apparently controlled by the external cation (Li(H2O)+ vs Mg(H2O)62+) coordinating to the urea O and pyridine N atoms and linking the capsules in their networks. Thus, the substitution of a bridging water molecule in C1 by an H2O-Li+ pair in C2 induces significant geometrical changes in the anion-binding

Urea-Functionalized Capsules with Tunable Anion-Binding Cavities

Crystal Growth & Design, Vol. 9, No. 4, 2009 1987

Table 1. Crystallographic Data for 1, 1a, and 1b

formula M cryst size (mm3) cryst syst space group a (Å) V (Å3) Z T (K) Fcalcd [g cm-3] 2θmax (deg) µ (cm-1) no. of reflns collected no. of independent reflns params Rint R1,a wR2b (I > 2σ(I)) a

1

1a

1b

C48H60Li2N20O12S 1155.10 0.20 × 0.17 × 0.15 cubic Pa3j 17.7907(7) 5630.9(4) 4 173(2) 1.363 50.04 0.136 29622 1668 133 0.0447 0.0621, 0.1678

C48H60Li2N20O12Se 1202.00 0.21 × 0.11 × 0.10 cubic Pa3j 17.9112(5) 5746.1(3) 4 173(2) 1.389 50.00 0.730 36590 1694 133 0.0639 0.0758, 0.2216

C48H60Li2N20O11S 1139.10 0.25 × 0.17 × 0.12 cubic Pa3j 17.7871(4) 5627.5(2) 4 173(2) 1.344 50.00 0.133 29421 1658 133 0.0447 0.0933, 0.2820

R1 ) ∑(|F0| - |Fc|)/∑|F0|. b wR2 ) {∑[w(F02 - Fc2)2]/∑[w(F02)2]}1/2.

Figure 3. Anion competition experiment. (a) Powder X-ray diffraction: simulated from single-crystal X-ray of 1 (black); experimental pattern from 1 (red); experimental pattern from the solid solution 2 obtained in the competition experiment (green). (b) FT-IR spectra of 1 (red) and 2 (blue).

site and thereby in the sulfate coordination mode. Essentially, this may be considered a form of allosteric regulation, commonly observed in proteins,8 but manifested here in an organic crystal.24 To assess the shape and size selectivity of C2, we set up a competition experiment between SO42-, SeO42-, SO32-, and CO32-. Like in the case of the analogous C1, we were interested to see whether C2 can recognize tetrahedral anions and discriminate against the pyramidal sulfite and trigonal-planar carbonate anions. More importantly, we asked whether the size discrimination of sulfate against selenate could be improved as a result of cavity reduction. Selenate and sulfite were found to form crystal structures (1a and 1b) that are isomorphous with 1, as determined by single-crystal X-ray crystallography. However, with carbonate we were not able to form an analogous complex, and only L1 · (H2O)2 crystals could be isolated under similar conditions. The structure of these crystals is very similar with that of another hydrate of L1, which was reported to form 1D chains through urea · · · urea hydrogen bonding.22 The fact that carbonate does not form a complex analogous to 1 could be attributed to the higher free energy of hydration corresponding to this anion,25 which made the hypothetical complex with L1 too soluble relative to the free ligand. Competitive crystallization of L1 (2 equivalents) with 1 equiv. of Li2SO4 and one equivalent of each SeO42-, SO32-, and CO32- (added as sodium

salts) in H2O/MeOH, resulted in a crystalline solid (2) with the same structure as 1, as indicated by its powder X-ray pattern (Figure 3a). FTIR spectroscopy showed that SO42- is the major anion in 2, with a small amount of SO32- also detectable (Figure 3b). Elemental analysis confirmed that 2 was a solid solution, with sulfate as the major anion and the other anions present in much smaller amounts: SO42-/SO32-/CO32-/SeO42- ) 36/4/2/1. The corresponding separation factors for sulfate against sulfite, carbonate, and selenate, are 14, 29, and 64, respectively. Notably, crystallization of 2 is also cation selective, with an observed Li+/Na+ separation factor of 9. It appears that the tetrahedral cation site in 2 favors Li+ over Na+, as the latter is known to prefer an octahedral coordination and is thus incompatible with the structure of 1. Separate experiments confirmed that Na2SO4 and L1 do not crystallize together to form a phase analogous to 1. However, the sodium cation apparently can be included in 1 in small amounts as an “impurity”. Overall, it can be considered that L1 acts as a selective ion-pair receptor through self-assembly26,27 in the crystalline state. Discussion The competition experiment established that C2 is capable of both shape recognition of sulfate against SO32-, and CO32-,

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Figure 4. Crystal structures of 1a and 1b. (a) SO32- binding site in 1b. Hydrogen bonds are shown as black dashed lines and repulsive interactions as red dashed lines. The sulfite is disordered over 6 positions (only one orientation shown), to emulate the S6 symmetry of the cage. (b) Overlay of X-ray crystal structures of 1 and 1a. SO42- structure is shown in magenta and SeO42- structure in green; the cations are omitted for clarity. Table 2. Geometric Parameters (Å, deg) for NH · · · O Hydrogen Bonds between Urea Groups and the Anions in 1, 1a, and 1b D-H · · · A

H· · ·A

D· · ·A

∠D-H-A

3.007 3.028 2.913 2.921

155.66 150.97 136.32 159.42

2.996 2.989 2.963 2.911

158.70 150.98 135.46 155.94

3.000 3.088 2.869 2.917

159.75 148.97 139.89 159.48

1 N2-H2N · · · O3 N2-H2N · · · O4 N3-H3N · · · O4

2.183 2.229 2.213 2.081

N2-H2N · · · O3 N2-H2N · · · O4 N3-H3N · · · O4

2.159 2.188 2.188 2.086

1a

1b N2-H2N · · · O3 N2-H2N · · · O4 N3-H3N · · · O4

2.159 2.300 2.140 2.076

and size recognition against SeO42-. The shape discrimination against sulfite can be rationalized from the crystal structure of 1b (Figure 4a), which shows that SO32- forms only 9 hydrogen bonds with the urea groups. Additionally, the sulfite anion is engaged in repulsive interactions (depicted as red dashed lines) between NH protons and the positively charged S atom, with dH · · · S contact distances of 2.86, 2.95 Å that are shorter than the corresponding sum of the van der Waals radii of 3.00 Å. Most remarkably, the selectivity of C2 for sulfate against selenate exceeds that observed in sulfate-binding protein.10 The size selectivity can be rationalized from the superimposed crystal structures of the capsule C2 with sulfate and selenate as included anions (Figure 4b), which show that C2 is relatively rigid in these crystals, despite the intrinsic conformational flexibility of its L1 constituents. This rigidity is most likely enforced by the Li(H2O)+ bridges, which maintain the cavity size and shape virtually unchanged regardless of the included anion. Most notably, the height of C2 only increases by 0.04 Å when going from sulfate to selenate, despite the fact that the Se-O bond is 0.16 Å longer than the S-O bond. Although the selenate anion sits lower inside the cavity by 0.06 Å compared with sulfate, the distance between the negatively charged tertiary N atom of L1 and the apical O atom of the anion is 0.05 Å shorter for selenate compared to sulfate (3.26 Å vs 3.31 Å). Although other more subtle geometrical differences in hydrogen bonding (Table 2) may play a role, this increasing repulsive interaction is likely a critical factor in the observed weaker binding of selenate. The size selectivity observed for C2 contrasts markedly with the behavior of the larger C1, which showed little discrimination

between these two similar anions. It may be thus considered that the anion binding sites in C2 and C1 are allosterically regulated by the external cations, Li(H2O)+ and Mg(H2O)62+, which control the internal structures of the capsules and thereby the anion selectivity. These results demonstrate that crystalline assemblies made from simple and flexible building blocks functionalized with a sufficient number of constrained hydrogen bonding groups may display exceptional anion recognition abilities comparable with those of protein receptors. Experimental Section All reagents and solvents were purchased from commercial suppliers and used without further purification. FT-IR spectra were recorded in KBr pellets with a Digilab FTS 7000 spectrometer. Powder X-ray diffraction patterns were obtained with a Bruker D5005 diffractometer using monochromatic Cu KR radiation (λ ) 1.5418 Å). Elemental analyses were performed by Galbraith Laboratories, Inc. Synthesis of L1. Nicotinyl azide13b (1.52 g, 10.26 mmol) was dissolved in 37 mL anhydrous benzene and the solution was heated to reflux for 5.5 h with magnetic stirring under an argon atmosphere. The benzene was removed on the rotavap, and the resulting m-pyridylisocyanate was dissolved in 37 mL dichloromethane. Tris(2-aminoethyl)amine (0.46 mL, 3.42 mmol) dissolved in 3 mL dichloromethane was added at once to the m-pyridylisocyanate solution, resulting in the formation of a white precipitate. The reaction mixture was stirred overnight under argon at room temperature, then the precipitate was filtered, washed with dichloromethane, and dried under a vacuum. Yield: 1.027 g (59%). 1H NMR (400 MHz, DMSO-d6, 25 °C): δ 8.72 (s, 3H; NH), 8.51 (s, 3H; CH), 8.08 (d, J ) 3.27 Hz, 3H; CH), 7.86 (d, J ) 4.42 Hz, 3H; CH), 7.21 (dd, J ) 4.63, 3.67 Hz, 3H; CH), 6.29 (t, J ) 5.22 Hz, 3H; NH), 3.20 (dt, J ) 6.14, 5.94 Hz, 6H; CH2), 2.58 (t, J ) 6.44 Hz, 6H, CH2). 13C NMR (100 MHz, DMSO-d6, 25 °C): δ 37.60, 53.78, 123.46, 124.49, 137.17, 139.59, 142.08, 155.23. A sample of analytical purity was obtained by recrystallization from water. Anal. Calcd (%) for L1 · 2.5H2O: C, 52.26; H, 6.40; N, 25.39. Found: C, 52.17; H, 6.03; N, 25.21. Synthesis of 1. L1 (0.101 g, 0.2 mmol) and Li2SO4 · H2O (0.013 g, 0.1 mmol) were dissolved in 9 mL of H2O/MeOH (2:7, v/v). Slow solvent evaporation resulted in crystals of 1, which were filtered and washed with water and methanol. Yield: 0.045 g (39%). Mp: 212-213 °C. FT-IR (KBr): ν ) 1101 cm-1 (SO42-). Anal. Calcd (%) for Li2SO4(L1)2(H2O)2: C, 49.74; H, 5.57; N, 24.17; S, 2.77; Li, 1.20. Found: C, 49.29; H, 5.54; N, 23.63; S, 2.77; Li, 1.17. Anion Competition Experiment. A solution of L1 (2.024 g, 4 mmol) in 50 mL of MeOH was mixed with 40 mL of an aqueous solution containing Li2SO4 · H2O (0.256 g, 2 mmol), Na2SO3 (0.252 g, 2 mmol), Na2CO3 (0.212 g, 2 mmol), and Na2SeO4 · 10H2O (0.738 g, 2 mmol). Slow solvent evaporation yielded crystals of 2 that were filtered and washed with water and methanol. Yield 1.253 g (54%). FT-IR (KBr): ν ) 1101 cm-1 (SO42-), 965 cm-1 (SO32-); Anal. Calcd. (%) for Li1.33Na0.67(SO4)0.828(SO3)0.099(CO3)0.05(SeO4)0.023(L1)2(H2O)2):

Urea-Functionalized Capsules with Tunable Anion-Binding Cavities C, 49.43; H, 5.53; N, 23.99; S, 2.55; Se, 0.16; Li, 0.79; Na, 1.32; sulfite, 0.68. Found: C, 49.11; H, 5.46; N, 23.57; S, 2.54; Se, 0.155; Li, 0.755; Na, 1.28; sulfite, 0.68. Separation factors: R(SO42-/X) ) [(mol SO42-)/(mol X)]crystal/[(mol SO42-)/(mol X)]solution were estimated on the basis of the measured yield of crystallization and the elemental composition of the isolated solid. X-Ray Crystallography. Single crystals of 1 were obtained by slow evaporation of a mixture containing L1 (0.2 mmol) and Li2SO4 (0.1 mmol) in 9 mL H2O/MeOH (2:7, v/v). Single crystals of 1a were obtained by slow evaporation of a mixture containing L1 (0.1 mmol) Na2SeO4 (0.05 mmol) and LiNO3 (0.1 mmol) in 4.5 mL H2O/MeOH (1:2, v/v). Single crystals of 1b were obtained by slow evaporation of a mixture containing L1 (0.1 mmol), Na2SO3 (0.5 mmol) and LiNO3 (1 mmol) in 6 mL of H2O/MeOH (1:1, v/v). Single-crystal X-ray data were collected on a Bruker SMART APEX CCD diffractometer with fine-focus Mo KR radiation (λ ) 0.71073 Å), operated at 50 kV and 30 mA. The structures were solved by direct methods and refined on F2 using the SHELXTL software package.28 Absorption corrections were applied using SADABS, part of the SHELXTL package. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in idealized positions and refined with a riding model. The water oxygen coordinating the Li+ sits on a S6 special position, so the corresponding water protons are disordered over 3 equivalent positions. Consequently, we could not locate the positions of these protons from the difference Fourier maps. The occurrence of water · · · pyridine hydrogen bonding was therefore inferred solely from the O · · · N intermolecular distance, which measured 3.157 Å in 1. In the case of the analogous 1a and 1b, which were grown from Li+/Na+ equimolar mixtures, the Li(H2O) units are disordered, presumably due to partial substitution of Li+ with Na+. The SO42- and SeO42- anions in 1 and 1a are disordered over two positions, while the SO32- anion in 1b is disordered over 6 positions, to emulate the S6 symmetry of the coordination cages. A summary of crystallographic data is listed in Table 1. Table 2 lists the hydrogen bonding parameters for anion binding in 1, 1a, and 1b.

Acknowledgment. This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract DE-AC05-00OR22725, with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.

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Supporting Information Available: X-ray crystallographic files (CIF). This material is available free of charge via the Internet at http:// pubs.acs.org.

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