ARTICLE pubs.acs.org/crystal
Crystallographic Implications for the Design of Halogen Bonding Anion Receptors Published as part of the Crystal Growth & Design virtual special issue on Halogen Bonding in Crystal Engineering: Fundamentals and Applications Nathan L. Kilah, Matthew D. Wise, and Paul D. Beer* Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, United Kingdom
bS Supporting Information ABSTRACT: X-ray crystallographic analysis has revealed the bis-halogen bond complexation of chloride and iodide by 4-(4-(tert-butyl)phenyl)-5-iodo-3-methyl-1-n-octyl-1H-1,2,3triazol-3-ium cations in two distinct coordination modes. The chloride complex displays a bent arrangement of the halogen bonding donors around the central anion (147.84(5) and 142.32(5)o), while the iodide salt adopts a structure with an angle closer to orthogonal (78.333(15)o). The X-ray crystal structure of a bromide salt of a halogen bonding rotaxane is also reported. The implications of the angle of halide coordination on the design of interlocked solution phase halogen bonding anion receptors are discussed.
’ INTRODUCTION Anions are ubiquitous and are of fundamental importance in many chemical, biological, medical, and environmental processes. Challenges arise when anions are present in large excess, such as the eutrophication of lakes;1,2 where they are misregulated, as observed in the chloride ion channels of cystic fibrosis sufferers;3 or where they are relevant to human health, such as the pertechnetate anion and the radioactive isotopes of iodide.4 The detection and quantification of anions in solution remain of pressing interest, and numerous synthetic receptors and sensors have been developed to tackle the diverse challenges posed by anions. These receptors make use of a large array of noncovalent interactions, including electrostatic, hydrogen bonding, Lewis acid base and anion-π interactions for their efficacy of operation in polar organic and aqueous solvent media.5 An underexploited noncovalent interaction in solution phase anion receptor chemistry is the halogen bond.6 This attractive interaction arises between a positively polarized halogen atom, frequently bromine or iodine, with a Lewis base. Anions are ideal halogen bond acceptors and are used extensively as templates for halogen bonding structures in the solid state.7 9 The development of solution phase anion receptors utilizing halogen bonding is in its infancy; however, the receptors reported to date show promising results in both their selectivity and efficacy in competitive polar and polar/aqueous solvents. The heteroditopic sodium halide receptor 1 was observed by 1H NMR spectroscopy competition experiments in CDCl3 to bind sodium iodide with high affinity, 20-fold greater than the perfluorinated analogue.10 The neutral anion receptor 2 incorporates convergent iodotetrafluorobenzene substituents and was shown by 19F NMR titrations r 2011 American Chemical Society
in acetone-d6 to favor the halides over oxoanions, with a clear preference for chloride.11 Additional investigations have demonstrated the influence of the aryl substitution pattern on the strength and selectivity of the anion association.12 The bis (2-bromoimidazolium) macrocyclic halogen bond chelate syn3.(PF6)2 was shown by 1H NMR spectroscopy titrations to selectively bind bromide over chloride and iodide as a 1:1 adduct in a competitive 9:1 CD3OD/D2O solvent mixture.13 This halogen bonding receptor displayed stronger and more selective
Received: June 28, 2011 Revised: August 1, 2011 Published: August 18, 2011 4565
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Crystal Growth & Design binding than the hydrogen bonding analogue. Mixed hydrogen and halogen bonding receptors, such as 4, have been shown to bind halides more strongly than oxoanions.14 The preference of the receptors for halides was attributed to the averaged contributions of the two types of noncovalent interactions. Recently, we have made efforts to incorporate halogen bond donor groups into structurally constrained anion receptors by exploiting the anion templation of three-dimensional interpenetrated and interlocked hosts.15,16 Limiting the degrees of freedom and increasing the steric constraints of anion receptors have been shown to increase both selectivity for and binding strength toward anions. Interlocked anion receptors, such as catenanes and rotaxanes, attempt to mimic the strength and specificity of anion binding performed in nature by the phosphate and sulfate binding proteins.17 19 These systems achieve strong, selective binding of their anionic substrates through the exclusion of solvent and shielding of the anion binding site deep within the hydrophobic protein core. In the course of our research program, we have identified the chloride anion-templated assembly of a 2-bromo-functionalized imidazolium-threading pseudorotaxane (5.Cl),15 and the bromide templated synthesis of the first halogen bonding rotaxane.16 The interlocked [2]rotaxane (6a. PF6) was observed to bind the halide anions with significantly higher affinity than the hydrogen bonding analogue (6b.PF6)20 and with a rare selectivity for iodide.
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of the observed coordination geometry and other structural features of halide halogen bonding on the design of interlocked, halogen bonding anion receptors are discussed.
’ RESULTS AND DISCUSSION 5-Halo-1,2,3-triazolium Salts. 5-Halo-1,2,3-triazolium cations are promising candidates for the development of halogen bonding anion receptors due to their synthetic versatility and the strong polarization of the halogen atom. 5-Iodo-1,2,3-triazoles can be readily prepared by a synthetically versatile modified copper catalyzed azide alkyne cycloaddition (CuAAC) reaction,21 while 5-bromo1,2,3-triazoles can be prepared by an alternate method, albeit with higher catalyst loading and longer reaction time.22 Regioselective methylation of triazoles can be readily accomplished with the use of trimethyloxonium tetrafluoroborate, in very high yields.16,20
Halogen Bonding in 7.X Salts. We have recently reported the synthesis and crystallization of the salts 7.X (where X = Cl, Br or I), which are stable and undergo no halogen exchange with their halide counterions.16 A representative ORTEP-3 ellipsoid plot of 7.Br is shown in Figure 1. Strong halogen bonds were identified in the three structures between the iodine atom of the 5-iodo1,2,3-triazolium cation and the halide ion (Table 1). The observed halogen bonding distances are among the shortest observed between iodine and the halides.
The halide in both the interpenetrated (5.Cl) and interlocked (6a.X where X = Cl, Br, or I) receptors is recognized by a combination of hydrogen and halogen bonding. The logical continuation of this field of research is the development of interlocked receptors interacting with the anionic guest solely through halogen bond donors. The development of such receptors requires the careful examination of the unique geometric and electronic properties of halogen bonding interactions, including the length of the halogen bond, the limitations of the highly linear nature of the interaction, the steric limitations of the bulky halogen atoms, and the stability of the halogen bond donor in the presence of nucleophiles. Fortunately, the large volume of crystallographic investigations present in the literature provides many useful insights into how to apply these fascinating noncovalent interactions in receptor structure design. Herein we report the crystal structures of two 5-iodo-1,2,3-triazolium salts where chloride and iodide are coordinated in significantly different geometries. The implications
Figure 1. X-ray structure of 7.Br. Ellipsoids are shown at 50%. Hydrogen atoms have been removed for clarity. Color scheme: carbon, gray; nitrogen, blue; bromide, orange; iodine, purple. 4566
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Table 1. Selected Distances (Å) and Angles (o) 7.Cla I3 3 3X
2.950(6)
7.Bra 3.0927(4)
7.Ia,b
[72.Cl]BF4c
[72.I]I3
6a.Bra
6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD
3.127(4)
3.052(2)
2.217(11)
2.071(14)
3.2833(8)
2.9903(15)
3.3496(6)
3.2818(8)
2.9500(15)
3.4079(6)
2.9744(14) 2.9772(14) C I
2.043(19)
2.098(3)
2.094(8)
2.088(4)
2.086(6)
2.129(9)
2.090(5)
2.102(7)
2.095(4) C I3 3 3X
176.9(5)
177.74(9)
179.2(2)
2.095(4) 174.90(14)
174.13(17)
179.1(2)
174.28(15)
174.72(18)
165.07(15)
175.2(4)
174.83(13) 174.95(13) I3 3 3X 3 3 3I
147.84(5)
78.333(15)
142.32(5) a
Previously reported.16 b Two molecules in the asymmetric unit. c Two molecules in the asymmetric unit related by pseudosymmetry.
The Cambridge Crystallographic Data Centre Structural Database (CSD) was analyzed for halogen bonding salts of chloride, bromide, and iodide to determine the common range of NM I 3 3 3 X interactions, where NM is a nonmetal and nonhalogen, and X is chloride, bromide, or iodide (Figure 2). This analysis demonstrates the correlation between the angle NM I 3 3 3 X and the length of the halogen bond: the more linear the arrangement of atoms, the stronger the interaction and the shorter the subsequent bond length. Similar scatter plot correlations have been reported for halogen bond acceptors including: nitrogen, oxygen, and sulfur;23,24 phosphorus, arsenic, and selenium;25 and the halides.8 Halogen Bonding in [72.X].Y Salts. Halide ions are remarkably flexible in their coordination number and can adopt many geometries including tetrahedral,26 square planar,27 29 and pentagonal. 30 In the course of preparing and characterizing the compounds 7.X (where X = Cl, Br, I), we were fortunate to obtain two samples where the halide ion was coordinated by two 5-iodo-1,2,3-triazolium cations in two distinct coordination geometries. The compound [72.Cl]BF4 crystallized in the monoclinic space group P21/c with two independent molecules in the asymmetric unit related by pseudosymmetry (Figure 3). The pseudosymmetry arises from the arrangement of the n-octyl chains in space. The central chloride atom in each molecule is coordinated by two halogen bonds at similar lengths to those observed for 7.Cl, approximately 80% of the sum of the van der Waals radii (Table 1).31 The two halogen bonds are positioned around the chloride in a bent geometry, with I 3 3 3 Cl 3 3 3 I angles of 147.84(5) and 142.32(5)o. The compound [72.I]I3 crystallized in the triclinic space group P1, with one molecule in the asymmetric unit (Figure 4). One octyl chain was disordered and was modeled over two sites of refined occupancy. The two 5-iodo-1,2,3triazolium groups are observed to halogen bond to a central iodide with halogen bonding distances approximately 85% of the sum of the van der Waals radii (Table 1).31 The two halogen bonds in [72.I]I3 are positioned around the iodide with an I 3 3 3 I 3 3 3 I angle of 78.333(15) Å, significantly less than orthogonal. The significant angular variation between the salts of [72.Cl]BF4 and [72.I]I3 was initially thought to arise from the crystal
Figure 2. Scatter plot of the halogen bonding distance I 3 3 3 X against the angle NM I 3 3 3 X (where NM is a nonmetal and nonhalogen and X is Cl , Br , or I shown in green, orange, and purple, respectively). Reported data were obtained from CSD version 5.32 May 2011, filtered to include only error-free, nondisordered structures with R < 0.075. Correlation values (R2): chloride, 0.4637; bromide, 0.3333; iodide, 0.2861.
packing effects of the cations with their tetrahedral (BF4 ) or linear (I3 ) counterions respectively, and the influence of additional noncovalent interactions to the anion within the crystal lattice. However, analysis of the CSD for salts of chloride, bromide, and iodide coordinated by two halogen bonds suggests a possible relationship between the halide ion and the geometry of coordination (Figure 5). This analysis indicated a preference of chloride and bromide for I 3 3 3 X 3 3 3 I angles closer to 180°, while the larger iodide ion adopts geometries ranging between 70 180°. A previously reported computational study of the interactions of bromide and iodide with two neutral halogen bond donors gave calculated I 3 3 3 X 3 3 3 I angles of ca. 100°.32 Halogen Bonding in the Interlocked Anion Receptor 6a. Br. In a recent publication, we disclosed receptor 6a.PF6, the first example of an interlocked anion receptor incorporating a halogen bonding donor group.16 Crystallographic analysis of 6a.Br was undertaken with synchrotron radiation, providing evidence for halogen bonding between the interlocked host and the bromide 4567
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Figure 3. X-ray structure of the asymmetric unit of [72.Cl]BF4. Ellipsoids are shown at 30%. Hydrogen atoms and the minor component of BF4 disorder have been removed for clarity. Color scheme: boron, orange; carbon, gray; nitrogen, blue; fluorine, light green; chloride, dark green; iodine, purple.
Figure 4. X-ray structure of the asymmetric unit of [72.I]I3. Ellipsoids are shown at 30%. Hydrogen atoms and the minor component of the octyl chain disorder have been removed for clarity. Color scheme: carbon, gray; nitrogen, blue; iodine, purple.
Figure 5. Histogram of angle NM I 3 3 3 X 3 3 3 I NM (where NM is a nonmetal and nonhalogen, and X is Cl , Br , or I shown in green, orange, and purple, respectively). Reported data were obtained from CSD version 5.32 May 2011 and include only error-free structures.
guest. This crystallographic analysis was complicated by small, weakly diffracting crystals which degraded in the X-ray beam. Crystals of 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD (Figure 6) were recently isolated, and the structure is presented here for comparison with 6a.Br. The bromide anion in both structures is coordinated by the iodine atom of the 5-iodo-1,2,3-triazolium group and the amide protons of the macrocyclic ring. A partial occupancy methanol was also observed to hydrogen bond to the bromide anion in 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD. The halogen bonds in both 6a.Br and 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD are 3.127(4) and 3.052(2), respectively (Table 1). The halogen bond angle C I 3 3 3 Br in 6a.Br is significantly deviated from linearity (165.07(15)o), while the equivalent angle in 6a. Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD (175.2(4)o) is closer to the angle in 7.Br. The distortion of the halogen bond away from a linear interaction in 6a.Br was attributed to the competitive hydrogen bonding contribution of the macrocyclic amides to the bromide. The halogen bond observed in 6a.Br 3 1.33CDCl3 3 0.68 4568
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Figure 6. X-ray structure of 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD. Ellipsoids are shown at 20%. Nonacidic hydrogen atoms and cocrystallized water and chloroform molecules have been removed for clarity. Color scheme: deuterium, white; carbon, gray; nitrogen, blue; oxygen, red; bromine, orange; iodine, purple.
D2O 3 0.62CD3OD is less deviated from linearity, possibly arising from the additional competitive hydrogen bonding interaction of the cocrystallized methanol solvate counterpoised to the two amide groups. The macrocyclic amide functional groups are roughly orthogonal to the halogen bond in both structures, with N(H) 3 3 3 Br 3 3 3 I angles observed in the range of 77.70 87.60° for 6a.Br and 77.01 and 77.94° for 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD. A preference for the orthogonal arrangement of halogen bond and hydrogen bond donors around a common carbonyl oxygen acceptor was observed for the solid state structure of proteins.33 The perpendicular arrangement of the noncovalent interactions was attributed to a combination of the in-plane and out-of-plane electronegative potential of the oxygen and the steric effects of the local peptide environment. A search of the CSD for mixed halogen and hydrogen bonding salts of chloride, bromide, and iodide returned many hits; however, no clear angular trend could be derived due to the large variation in the number of noncovalent interactions present in the individual structures. Presumably, the more diffuse electron clouds of the halides offer greater angular flexibility than the orthogonal relationship observed for carbonyl oxygen atoms in biological systems. Therefore, the steric contributions of the interlocked structure are most relevant in the halogen bond halide hydrogen bond angles observed in 6a.Br and 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD. Further noncovalent interactions in the interlocked structure were observed between the 5-iodo-1,2,3-triazolium axle and the macrocycle, including π π stacking between the electron deficient triazolium ring and the electron rich hydroquinones and secondary hydrogen bonding between the triazolium methyl group and the polyether chain of the macrocycle. Implications for Anion Receptor Design. The crystal structures presented above and the subsequent CSD searches highlight a number of structural properties relevant for future halogen bonding anion receptor design. Halogen bonds are frequently compared to hydrogen bonds in their strength and application, but the most striking difference between these two noncovalent interactions is their angle of interaction. Hydrogen bonds are formed with a range of angles, while halogen bonds are, by their nature, linear. The crystal structures of 7.X (where X = Cl, Br, or I) display linear halogen bonds, while the equivalent bonds in [72.Cl]BF4 and [72.I]I3 are somewhat deviated from linear
Figure 7. Approximation of the angle θ for a chloride bound by two iodine halogen bond donors.
(Table 1). This deviation may arise from the steric influence of the close proximity 5-iodo-1,2,3-triazolium groups and their substituents. The linearity of halogen bonds can also be influenced by neighboring groups, as observed in the structure of 6a.Br where the macrocyclic amides compete for the bromide guest and in the structure of 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62 CD3OD where the influence of a cocrystallized methanol solvate appears to influence the halogen bond linearity. The observed geometry around the chloride and iodide ions in [72.Cl]BF4 and [72.I]I3, the subsequent CSD search results (Figure 5), and the mixed halogen hydrogen bonding motif in the structures of 6a.Br raise interesting questions as to the coordinative preference of halides in multiple halogen bonding systems. The preference of bis-halogen bond coordination I 3 3 3 X 3 3 3 I angles for chloride and bromide of approximately 180° and the relative angular freedom of iodide arise from a combination of steric and electronic contributions. Chloride and bromide are less electronically diffuse than iodide and form shorter halogen bonds to iodine halogen bond donors. Decreasing the angle I 3 3 3 X 3 3 3 I around these compact anions would bring the halogen bond donor iodine atoms to a distance approaching the sum of their van der Waals radii, increasing repulsion and disfavoring smaller angles. A simple trigonometry calculation can be applied to estimate a “minimum angle of approach”. Given the sum of the iodine van der Waals radii (3.96 Å) and an average I 3 3 3 Cl bond length from [72.Cl]BF4 (2.97 Å), this angle (θ) would be ca. 84° (Figure 7). In reality, the structures summarized in Figure 5 possessing I 3 3 3 Cl 3 3 3 I angles 2.0σ(I))
17468
8728
7760
θ range, °
5.1 27.5
5.1 27.5
5.1 23.3
temperature, K
150
150
150
final R, wR (I > 2.0σ(I))
0.0587, 0.0699
0.0458, 0.0716
0.1143, 0.2602
R, wR (all data)
0.0763, 0.0767
0.0723, 0.0902
0.1551, 0.2955
1.92,
2.08,
1.80,
Dcalcd, g cm μ, mm GOF
3
1
largest diff peak and hole, e Å
3
1.90
The longer halogen bonding lengths to the iodide anion, however, positions the donor iodine atoms further away, thus limiting the repulsion between the bulky donor groups and allowing for a wider range of angles between the halogen bonds. For example, in [72.I]I3 the interatomic distance of the two iodine halogen bond donor atoms is 4.268(1) Å, considerably larger than the sum of the van der Waals radii, with an I 3 3 3 I 3 3 3 I angle of 78.333(15)o. The geometric effects of halogen bonding on anion receptor selectivity, with particular relevance to the bis-coordination of halides by halogen bonds, are readily observed in the structure of syn-3. Br2.13 This receptor shows selectivity for bromide over chloride and iodide, and it is the unique geometric properties of the molecule which imparts its selectively. Significant steric crowding is observed in the structure of syn-3.Br2, where the two halogen bond donor groups are within the sum of their van der Waals radii (ca. 96%) and chelate the bromide with a Br 3 3 3 Br 3 3 3 Br angle of 67.15°.13 The close proximity of the donor bromine atoms in space and the acute angle formed between them necessitate deviation of the halogen bond angle away from linearity (166.81(12)o). Thus, selectivity of this anion receptor most likely arises from the combination of the geometric parameters: the shorter halogen bond lengths of the chloride adduct would necessitate increased displacement of the halogen bond angle away from 180° while simultaneously bringing the bromine donor atoms too close together, while the longer bond lengths required by the iodide adduct would increase the distance between the bromine halogen bond donors, limiting their ability to form a stable chelate. The angular preference observed for the bis coordinated halide in [72.Cl]BF4 and [72.I]I3 raises further questions regarding the anion coordinating geometry of the interlocked halogen bonding anion receptor 6a.PF6. Previously, we attributed the preference
2.61
1.22
of the interlocked receptor for iodide to the accessibility of the binding site to larger anions and to weaker competition for the more lipophilic halides in an aqueous solvent system.16 In light of the influence of the steric and electronic properties of the bulky halogen bond donor atom, and the observed angular preference for combinations of hydrogen and halogen bonds around carbonyl oxygen groups,33 it is possible that the anion binding pocket of 6a+ and the orthogonal binding mode of the mixed halogen bond isophthalamide pocket is more suitable for the binding of iodide than chloride and bromide. The interplay of the geometric properties observed in syn-3. Br2, [72.Cl]BF4, [72.I]I3 and the structures of 6a.Br provide significant insights for the future design of interlocked anion receptors. The interface between the components of interlocked anion receptors such as [2]catenanes34,35 and [2]rotaxanes16,20,36 is crucial for both their selectivity and strength of anion binding. The incorporation of halogen bonds into these constrained clefts in either orthogonal or linear arrays could potentially be applied for selective binding of halides.
’ CONCLUSION X-ray crystallography is an invaluable tool for the design and understanding of solution-phase halogen bonding anion receptors. 5-Iodo-1,2,3-triazolium salts are promising candidates for the development of halogen bonding anion receptors, as demonstrated by the strong halogen bonding interactions observed for the chloride, bromide, and iodide salts of 7+ in the solid state. Distinct halide coordination geometries were observed for the salts [72.Cl]BF4 and [72.I]I3 (147.84(5) and 142.32(5)o for chloride and 78.333(15)o for iodide), and examination of the Cambridge Structural Database suggested an underlying preference of the halides for their observed 4570
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Crystal Growth & Design geometries. The future rational design of tunable and selective interlocked halogen bonding anion receptors will be well served by the consideration of the length, linearity, and chelate angle of halogen bonding interactions.
’ EXPERIMENTAL SECTION Synthesis. The synthesis of 7.Cl, 7.Br, 7.I, and 6a.Br have been reported previously.16 Crystals of [72.I].I3 were obtained by spontaneous oxidation of a solution of 7.I in dichloromethane/diisopropyl ether. Crystals of [72.Cl]BF4 were obtained by slow diffusion of diisopropylether into a dichloromethane solution of 7.BF4 which had undergone partial anion exchange with aqueous ammonium chloride solution. Crystals of 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD were obtained from slow evaporation of a 45:45:10 CDCl3/CD3OD/D2O solution of 6a.PF6 containing 10 equiv of TBABr. X-ray Crystallography. Single crystal X-ray diffraction data for [72.Cl]BF4, [72.I].I3, and 6a.Br 3 1.33CDCl3 3 0.68D2O 3 0.62CD3OD were collected using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) on a Nonius KappaCCD diffractometer. The diffractometer was equipped with a Cryostream N2 open-flow cooling device,37 and the data were collected at 150(2) K. Series of ω-scans were performed to a maximum resolution of 0.77 Å. Cell parameters and intensity data (including interframe scaling) were processed using the DENZO-SMN package.38 The structures were solved by direct methods using SIR92,39 or by charge flipping methods with Superflip,40 and refined using full-matrix least-squares on F2 within the CRYSTALS suite.41 Molecular graphics were produced with ORTEP-342 and were rendered in POVray.43 Crystal data, data collection parameters, and analysis statistics are given in Table 2.
’ ASSOCIATED CONTENT
bS
Supporting Information. Crystallographic information files (CIFs) are available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: +44 (0) 1865 285142; fax: +44 (0) 1865 272690; e-mail:
[email protected]; Web: http://research.chem.ox.ac.uk/ paul-beer.aspx.
’ ACKNOWLEDGMENT N.L.K. thanks the Royal Commission for the Exhibition of 1851 for a research fellowship. We acknowledge Oxford Chemical Crystallography for the use of their instruments and Dr. Amber Thompson for helpful discussions. ’ REFERENCES (1) Mason, C. F. Biology of Freshwater Pollution, 2nd ed.; Longman Scientific & Technical: Harlow, 1991. (2) Moss, B. Chem. Ind. 1996, 11, 407–411. (3) Akabas, M. H. J. Biol. Chem. 2000, 275, 3729–3732. (4) Ryo, U. Y.; Vaidya, P. V.; Schneider, A. B.; Bekerman, C.; Pinsky, S. M. Radiology 1983, 148, 819–822. (5) Sessler, J. L.; Gale, P. A.; Cho, W.-S. Anion Receptor Chemistry; RSC Publishing: Cambridge, 2006. (6) Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. J. Mol. Model. 2007, 13, 291–296. (7) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem., Int. Ed. 2008, 47, 6114–6127.
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(8) Metrangolo, P.; Pilati, T.; Terraneo, G.; Biella, S.; Resnati, G. CrystEngComm 2009, 11, 1187–1196. (9) Custelcean, R. Chem. Soc. Rev. 2010, 39, 3675–3685. (10) Mele, A.; Metrangolo, P.; Neukirch, H.; Pilati, T.; Resnati, G. J. Am. Chem. Soc. 2005, 127, 14972–14973. (11) Sarwar, M. G.; Dragisic, B.; Sagoo, S.; Taylor, M. S. Angew. Chem., Int. Ed. 2010, 49, 1674–1677. (12) Dimitrijevic, E.; Kvak, O.; Taylor, M. S. Chem. Commun. 2010, 46, 9025–9027. (13) Caballero, A.; White, N. G.; Beer, P. D. Angew. Chem., Int. Ed. 2011, 50, 1845–1848. (14) Chudzinski, M. G.; McClary, C. A.; Taylor, M. S. J. Am. Chem. Soc. 2011, 133, 10559–10567. (15) Serpell, C. J.; Kilah, N. L.; Costa, P. J.; Felix, V.; Beer, P. D. Angew. Chem., Int. Ed. 2010, 49, 5322–5326. (16) Kilah, N. L.; Wise, M. D.; Serpell, C. J.; Thompson, A. L.; White, N. G.; Christensen, K. E.; Beer, P. D. J. Am. Chem. Soc. 2010, 132, 11893–11895. (17) Pflugrath, J. W.; Quiocho, F. A. Nature 1985, 314, 257–260. (18) Luecke, H.; Quiocho, F. A. Nature 1990, 347, 402–406. (19) He, J. J.; Quiocho, F. A. Science 1991, 251, 1479–1481. (20) Mullen, K. M.; Mercurio, J.; Serpell, C. J.; Beer, P. D. Angew. Chem., Int. Ed. 2009, 48, 4781–4784. (21) Hein, J. E.; Tripp, J. C.; Krasnova, L. B.; Sharpless, K. B.; Fokin, V. V. Angew. Chem., Int. Ed. 2009, 48, 8018–8021. (22) Kuijpers, B. H. M.; Dijkmans, G. C. T.; Groothuys, S.; Quaedflieg, P. J. L. M.; Blaauw, R. H.; van Delft, F. L.; Rutjes, F. P. J. T. Synlett 2005, 20, 3059–3062. (23) Lommerse, J. P. M.; Stone, A. J.; Taylor, R.; Allen, F. H. J. Am. Chem. Soc. 1996, 118, 3108–3116. (24) Rissanen, K. CrystEngComm 2008, 10, 1107–1113. (25) Ouvrard, C.; Le Questel, J. Y.; Berthelot, M.; Laurence, C. Acta Crystallogr., Sect. B: Struct. Sci 2003, 59, 512–526. (26) Wisner, J. A.; Beer, P. D.; Drew, M. G. B.; Sambrook, M. R. J. Am. Chem. Soc. 2002, 124, 12469–12476. (27) Yang, X.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem., Int. Ed. 1991, 30, 1507–1508. (28) Zheng, Z.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc. 1995, 117, 5105–5113. (29) Wielandt, J. W.; Kilah, N. L.; Willis, A. C.; Wild, S. B. Chem. Commun. 2006, 3679–3680. (30) Duer, M. J.; García, F.; Kowenicki, R. A.; Naseri, V.; McPartlin, M.; Stead, M. L.; Stein, R. S.; Wright, D. S. Angew. Chem., Int. Ed. 2005, 44, 5729–5733. (31) Bondi, A. J. Phys. Chem. 1964, 68, 441–452. (32) Shi, Q.-C.; Lu, Y.-X.; Fan, J.-C.; Zou, J.-W.; Wang, Y.-H. J. Mol. Struct.: THEOCHEM 2008, 853, 39–44. (33) Voth, A. R.; Khuu, P.; Oishi, K.; Ho, P. S. Nat. Chem. 2009, 1, 74–79. (34) Huang, B.; Santos, S. M.; Felix, V.; Beer, P. D. Chem. Commun. 2008, 4610–4612. (35) Hancock, L. M.; Gilday, L. C.; Kilah, N. L.; Serpell, C. J.; Beer, P. D. Chem. Commun. 2011, 47, 1725–1727. (36) Hancock, L. M.; Gilday, L. C.; Carvalho, S.; Costa, P. J.; Felix, V.; Serpell, C. J.; Kilah, N. L.; Beer, P. D. Chem.—Eur. J. 2010, 16, 13082–13094. (37) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105–107. (38) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307–326. (39) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (40) Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40, 786–790. (41) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487. (42) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. (43) Persistence of Vision (TM) Raytracer; Persistence of Vision Pty. Ltd.: Williamstown, Victoria, Australia, 2004.
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