Solvent-Modulated Formation of “Pac-man” and Capsular Host

Article pubs.acs.org/crystal

Solvent-Modulated Formation of “Pac-man” and Capsular Host− Guest Bilayers from a Dicationic Ionic Liquid and C‑Butylpyrogallol[4]arene Drew A. Fowler,† Constance R. Pfeiffer,† Simon J. Teat,‡ Gary A. Baker,*,† and Jerry L. Atwood*,† †

Department of Chemistry, University of MissouriColumbia, 601 South College Avenue, Columbia, Missouri 65211, United States Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS6R2100, Berkeley, California 94720, United States

‡

S Supporting Information *

ABSTRACT: The pyrogallol[4]arenes have been shown to act as versatile host macrocycles for a wide variety of guest molecules, including the imidazolium-based cations of ionic liquids. This report demonstrates the use of alkyl-linked geminal dications in the design of bilayers comprising dimeric host−guest complexes. The pivotal role of solvent choice in controlling the resultant solid-state structure is particularly highlighted. The synthesis and single-crystal X-ray diffraction structures of two dimeric host−guest cocrystals generated by the use of different solvents while employing identical host and guest species are presented to illustrate this point. In one solvent, a “Pac-man”-type dimeric host−guest complex is assembled into bilayer galleries. With a switch to an alternate crystallization solvent, a bilayer-type structure in which each layer is composed of alternating complexes of a capsule and two “offset dimers” is instead constructed.

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INTRODUCTION Pyrogallol[4]arenes are widely studied macrocycles within the field of supramolecular chemistry. These bowl-shaped molecules are highly versatile units because of the 12 hydroxyl groups occupying their upper rims and the alkyl-chain functionality decorating their lower rims. The synthetic flexibility of these macrocycles leads to diverse and interesting supramolecular assemblies, including nanocapsules, nanotubes, and metal-seamed dimers.1−3 Numerous guest species have already been cocrystallized with pyrogallol[4]arenes, including small inorganics, drug molecules, and fluorescent probes.4−6 Most recently, ionic liquids have been included as guests to generate interesting and structurally informative supramolecular assemblies. Ionic liquids (ILs) have recently emerged as potentially ecofriendly designer solvents. The intensive research efforts surrounding these emergent solvents stem from a number of favorable properties such as negligible vapor pressure, high thermal stability, and the facility with which these salts can be synthetically tailored, leading to easily modulated properties.7 Indeed, ILs currently enjoy wide application in a variety of fields ranging from nanomaterials synthesis and tribology to biomass processing and energy applications.8 Our research groups have jointly begun to explore the use of ILs as a guest toolkit to tailor host−guest assemblies derived from pyrogallol[4]arene (PgCx) macrocycles.9 To date, reports on the host−guest chemistry of ILs remain fairly limited, however. This is especially true with regard to the calixarene macrocycle family to which the pyrogallol[4]arenes belong. Imidazolium© 2014 American Chemical Society

based cations account for the largest portion of reported ILs and have been the primary targets of studies with calixarenes. For example, the Raston and Alias research groups have examined the use of the water-soluble p-sulfonatocalix[4]arene as a host compound for imidazolium ions in the construction of supramolecular architectures and the stabilization of fullerene suspensions.10 Of particular relevance to the work presented here is our recent report on a cocrystal comprising a complex of a bis(imidazolium) IL hosted by a pyrogallol[4]arene macrocycles to yield a bilayer structure formed from alternately arranged dimeric host−guest complexes.9a The work presented here builds on this earlier result by showing that novel crystallization architectures can be achieved within bis(imidazolium) IL-derived cocrystals by simple solvent-modulated crystal engineering. When solvent molecules are present within crystal structures, they can often play important roles in the assemblies formed and the overall packing arrangement. The role of the crystallization solvent is particularly evident in the two new cocrystals reported here. In both cocrystals, [C3(C1Im)2]Br2 forms self-assembled dimeric assemblies with PgC4, and the resultant structures of the two cocrystals, one synthesized in acetone and the other in methanol, differ dramatically from one another. Herein, we present two new crystal structures for cocrystals composed of PgC4 (Figure 1a) and the dicationic IL Received: May 30, 2014 Revised: June 24, 2014 Published: July 7, 2014 4199

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Table 1. Comparison of Unit Cell Parameters for Isostructural Cocrystals of PgC2 and [C3(C1Im)2]Br2 crystallization solvent unit cell dimensions

Figure 1. Schematic structures of (a) a generic C-alkylpyrogallol[4]arene (PgCx) and (b) 3,3′-(propane-1,3-diyl)bis(1-methyl-1H-imidazol-3-ium) {[C3(C1Im)2]Br2}. volume (Å3) space group

3,3′-(propane-1,3-diyl)bis(1-methyl-1H-imidazol-3-ium) dibromide {[C3(C1Im)2]Br2 (Figure 1b)}. To emphasize the fact that solvent choice does not universally modulate the solid-state structure, we also present the formation of a cocrystal comprising PgC2 and [C3(C1Im)2]Br2 units that is isostructural to our previously reported cocrystal but was achieved by crystallization from a different solvent.

PgC2

[C3(C1Im)2]Br2

methanol (ref 9a) a = 34.618(10) Å b = 10.229(3) Å c = 22.447(7) Å β = 92.266(4)° 7942(4) C2/c

acetonitrile (cocrystal 1) a = 34.430(3) Å b = 10.178(1) Å c = 22.398(2) Å β = 92.226(2)° 7842(1) C2/c

absent from the structures. This plays a pivotal role in the resulting isostructural nature of the two crystalline products. Cocrystal 2. Crystallization of PgC4 with [C3(C1Im)2]Br2 in a 9:1 acetone/water solution results in the formation of a “Pacman”-shaped dimeric host−guest capsule (see Figure 3a,b). The resultant cocrystal 2 contains in the asymmetric unit one PgC4, one-half of a [C3(C1Im)2]2+ cation, one bromide ion disordered over three positions, and three and one-half water molecules disordered over four positions. In this instance, as in the PgC2 cocrystals discussed earlier, the primary solvent is

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RESULTS Cocrystal 1. Previously, we reported the synthesis and single-crystal X-ray diffraction structure of a dimeric host−guest complex comprised of PgC2 and [C3(C1Im)2]Br2.9a This complex was crystallized from a solution of methanol. The same offset dimeric host−guest complex has now been crystallized from a solution of acetonitrile, resulting in an isostructural solid-state structure, as shown in Figure 2.

Figure 2. Cocrystal 1 showing two PgC2 units, the [C3(C1Im)2]2+ dication and the C−H···π interactions present. Hydrogen atoms, Br− ions, and solvent molecules have been omitted for the sake of clarity.

A comparison of the two crystal structure unit cell parameters can be found in Table 1. With the exception of one added C−H···π interaction replacing a C−H···O hydrogen bond, the noncovalent host− guest interactions in the complex occur at identical positions. The addition of a fourth C−H···π interaction in place of a C− H···O hydrogen bond is due to the hydrogen atoms donated from a methyl group being slightly rotated. This results in a more favorable interaction with the aromatic ring over the phenolic oxygen atom. The only divergence in the synthesis of this cocrystal and the previously reported cocrystal is the solvent employed in the crystallization, acetonitrile in place of methanol. In both cocrystals, the principal solvents are notably

Figure 3. (a and b) Alternate views of the “Pac-man” complex of PgC4 and [C3(C1Im)2]2+. PgC4 molecules are colored yellow and [C3(C1Im)2]2+ ions white in space-filling representation. Tails, water molecules, and Br− ions have been omitted for the sake of clarity. (c) Packing of cocrystal 2 with PgC4 molecules in stick representation and [C3(C1Im)2]2+ (green) in space-filling representation. Water molecules and Br− ions have been omitted for the sake of clarity. 4200

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donated to disordered water molecules with O···O distances in the range of 2.67−2.88 Å. The three remaining hydrogen bonds are intramolecular hydrogen bonds around the upper rim of the macrocycle and have O···O distances of 2.70, 2.74, and 2.78 Å. Cocrystal 3. The result of crystallizing PgC4 with [C3(C1Im)2]Br2 in methanol is a host−guest cocrystal that contains three PgC4 macrocycles, 1.5 [C3(C1Im)2]2+ cations, three Br− ions disordered over six positions, 3.45 methanol molecules disordered over five positions, and 7.3 water molecules disordered over 11 positions in the asymmetric unit. This cocrystal (3) is particularly interesting because of the formation of two distinct dimeric host−guest assemblies. The two host−guest complexes are a dimeric capsule (3a) and an offset dimer (3b) (Figure 5a,b).

absent from the cocrystal and the host−guest complex is crystallized with the counterions and disordered water molecules. Despite this similarity, cocrystal 2 presents a markedly different structure, specifically with regard to the host−guest complex formed. A simple expansion of the structure due to an elongation in tail length from ethyl to butyl could be the envisioned reason for this structural difference. This turns out not to be the case, however. As is the case for the product of PgC2 and [C3(C1Im)2]Br2,9a the current host−guest complex is formed by each imidazolium ring of the dication being complexed by a macrocycle. In marked contrast, however, the dication has twisted around in such a way that the host−guest complex formed more closely resembles a capsulelike entity rather than the offset dimer seen in our previously reported structure. These dimeric capsules pack into a bilayer-type structure in which the disordered bromide ions and water molecules are positioned around the upper rims of the macrocycles and are therefore able to participate in hydrogen bonding with the phenolic oxygens of the PgC4 molecules. This arrangement is shown in Figure 3c. There are four symmetry-unique noncovalent host−guest interactions in the complex found in cocrystal 2, which includes eight of the 18 total hydrogen atoms of the dication interacting with the two PgC4 host molecules (see Figure 4).

Figure 4. Alternate views of the noncovalent interactions involved in the host−guest complex of cocrystal 2.

These occur through three C−H···O hydrogen bonds and one C−H···π interaction. Two of the C−H···O hydrogen bonds are donated from the propyl linker to the oxygen atoms’ upper-rim phenolic groups and have C−H···O distances of 2.64 and 2.78 Å. The third C−H···O hydrogen bond is donated from the methyl group to the upper rim with a C−H···O distance of 2.68 Å. The C−H···π interaction occurs through the donation of a hydrogen atom from the imidazolium ring to a benzene ring of the macrocycle and has a C−H···π aromatic centroid distance of 2.59 Å. Because of the positions of both the water molecules and the bromide counterions, the hydrogen bonding of the upper-rim hydroxyl groups of the PgC4 host molecules have multiple positions that likely occur in the cocrystal. The following discussion of the hydrogen bonding occurring in cocrystal 2 will therefore focus on the most sensible and primary positions based on the D···A distance and occupancy of the disordered atoms. Of the 12 upper-rim hydroxyl groups in the asymmetric unit, three of these are donating hydrogen bonds to bromide counterions. These three interactions have O···Br− distances of 3.01, 3.11, and 3.12 Å. There are six hydrogen bonds being

Figure 5. (a) Dimeric capsule (3a) found in cocrystal 3. (b) Offset dimer host−guest complex (3b) found in cocrystal 3. The dicationic guest is shown in space-filling representation. (c) Packing pattern of cocrystal 3. The guest molecules of 3a are colored blue in space-filling representation, and the guest molecules of 3b are colored green in space-filling representation. Solvent molecules and counterions have been omitted for the sake of clarity.

The complexes pack into a bilayer-type structure in which each layer is composed of alternating complexes with a ratio of one capsule to two offset dimers (Figure 5c). Unlike the “Pac-man” dimer of cocrystal 2, the two PgC4 macrocycles in 3a are not angled toward one side but rather are related by an inversion center, giving the host−guest complex a more capsular shape (see Figure 6a,b). The host−guest complex in 3a has 11 noncovalent host− guest interactions in total (see Figure 6c,d). Two of these are symmetry-related C−H···π interactions between a hydrogen atom of the imidazolium rings and a benzene ring of the PgC4 molecules with a C−H···π distance of 2.70 Å. There are also 4201

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to each another than to the nearest aromatic ring of the PgC4 molecules, which shows a π···π aromatic centroid distance of 3.81 Å. This indicates that there may still be some stacking interaction present here between the imidazolium IL headgroup and the aryl ring of PgC4. As we mentioned earlier, one would intuitively consider this a more favorable π···π stacking interaction because the imidazolium rings are positively charged and the aromatic rings of PgC4 donate electron density from the three hydroxyl groups to the ring. The second dimeric host−guest complex of cocrystal 3 has a shape quite different from that of either of the previously mentioned complexes. In 3b, the complex is an “offset dimer”, but rather than stacking in a linear fashion, the two macrocycles are twisted almost orthogonally as illustrated in Figure 6. There are 12 noncovalent interactions between the guest and the two host molecules in 3b (Figure 6c,d). One of the imidazolium ions is disordered equally over two positions, resulting in the loss of two C−H···O interactions between the methyl group and the macrocycle. These two C−H···O hydrogen bonds have H···O distances of 2.84 and 2.98 Å. Additionally, there are four C−H···O hydrogen bonds with distances in the range of 2.50− 2.80 Å. Further, there are six C−H···π interactions with H···π aromatic centroid distances in the range of 2.50−2.94 Å. Host− guest complex 3b contains two symmetry-unique PgC4 molecules, resulting in 24 unique hydroxyl groups that participate in hydrogen bonding. Similar to the hydrogen bonding interactions already discussed, the focus will again center on D···A distances and occupancies for the disordered atoms. In 3b, there are five hydrogen bonds donated to Br− counterions with O···Br− distances in the range of 3.10−3.15 Å. There are two hydrogen bonds donated to water molecules with O···O distances of 2.72 and 2.85 Å. There are also two hydrogen bonds donated to the hydroxyl groups of methanol solvent molecules, which have O···O distances of 2.57 and 2.88 Å. Furthermore, there are four intermolecular hydrogen bonds between the upper rims of neighboring PgC4 molecules that have O···O distances in the range of 2.69−2.94 Å. The 11 remaining hydroxyls participate in intramolecular hydrogen bonds around the upper rims of the two macrocycles with O··· O distances in the range of 2.65−2.91 Å.

Figure 6. (a and b) Alternate views of 3a with the guest cation colored blue in space-filling representation. (c and d) Alternate views of 3a with the noncovalent host−guest interactions shown as dashed bonds. The tail groups have been omitted for the sake of clarity.

two symmetry-related C−H···O hydrogen bonds in which the C2 hydrogen atoms of the imidazolium rings are donated to an upper-rim phenol. These interactions have C−H···O distances of 2.29 Å. Each of the two methyl groups is donating two hydrogen atoms in C−H···O hydrogen bonding and has C− H···O distances of 2.42, 2.42, 2.77, and 2.90 Å. The three remaining host−guest interactions all occur by donation of hydrogen atoms from the propyl linker to upper-rim phenols and have C−H···O distances of 2.79, 2.82, and 2.96 Å. Again, the hydrogen bonding in this structure is disordered, and we therefore focus on the most plausible and primary positions based on D···A distances and occupancies of the disordered atoms. In 3a, there is a single PgC4 in the asymmetric unit. The majority of the upper-rim hydroxyl groups of this host (i.e., eight) participate in intramolecular hydrogen bonding. These hydrogen bonds have O···O lengths in the range of 2.68−2.73 Å. Of those remaining, four hydroxyl groups form hydrogen bonds to two Br− counterions with O··· Br− distances of 2.93 and 3.01 Å. The last two form hydrogen bonds to water molecules and have O···O distances of 2.52 and 2.67 Å. One particularly interesting characteristic of 3a is the conformation of the guest [C3(C1Im)2]2+ cation. The two imidazolium moieties have twisted around such that they are within a reasonable distance and relative positions to be interacting with each other through π···π stacking. Indeed, the π···π aromatic centroid distances of the two imidazolium rings are 3.55 Å apart. This conformation was somewhat unexpected because these species are charged. A much more anticipated result would have been for the imidazolium rings to interact more closely with an aromatic ring of the macrocycle. We might even have predicted a deformation in the overall shape of the host−guest complex because of repulsive interactions, such as the one observed already for the “Pac-man” dimer in which the imidazolium rings slip away from one another. It is also worth noting that the two imidazolium rings are in fact closer

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DISCUSSION These results have yielded new cocrystals of PgCxs with a number of interesting properties. The cocrystals presented display how significantly the change in or presence of a particular crystallization solvent modulates the host−guest complex formed and the overall structure of the cocrystal. Formation of isostructural cocrystals from different crystallization solvents stems, in large part, from the solvent being absent from the crystal. The different crystalline architectures of cocrystals 2 and 3 illustrate just how significantly the crystallization solvent can impact the solid-state structure of cocrystals formed from identical host−guest components. PgCxs have significant flexibility in their bowl-shaped structure, with the two extremes being represented by a C4v bowl and a C2v pinched cone. To determine if the guest positions and packing within the solid-state structure have an effect on the shape of the macrocycles, we have measured crosssectional distances for each macrocycle by using calculated πaromatic centroids for each benzene ring. In cocrystal 2, the single asymmetric PgC4 has dimensions of 5.99 Å × 7.36 Å. The asymmetric macrocycle in 3a has dimensions of 6.37 Å × 7.13 Å, and the two asymmetric macrocycles in 3b exhibit 4202

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Crystal Growth & Design dimensions of 6.45 Å × 7.15 Å and 6.21 Å × 7.31 Å in the crystal structure. In cocrystal 3, all three PgC4 host molecules are associated with a different guest position. In 3a, the C4−C5 bond of the imidazolium ring is positioned deepest within the cavity. In 3b, the two PgC4 molecules have a methyl group and the propyl linker positioned deepest within the cavities. Yet all three macrocycles retain very similar shapes. This outcome reveals that, despite quite different structures and guest positions, the shape of the macrocycle is very accommodating and does not vary significantly. Cocrystal 3 is particularly interesting because of both the previously mentioned π-stacking interactions in 3a and the cocrystallization of two host−guest complexes in which the guest molecules occupy different positional geometries. Earlier, we reported on the synthesis of a bilayer-type cocrystal in which the host−guest complexes segregate into discrete bilayers based upon the guest position.9b In cocrystal 3, the structural differences are significantly more pronounced. This is largely due to the tethered nature of the dicationic guest molecule. In our previously reported structure, it was apparent that the conformation of the guest affected the packing of the host− guest complexes due to the formation of an AB-type bilayer structure. While the two different host−guest complexes of 3 do not crystallize into distinct layers, the effect of the tether is very pronounced, as evidenced by how different the dimeric complex shape becomes (i.e., an L-shaped offset dimer vs a dimeric capsule).

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Article

Synthesis of Pyrogallol[4]arene. All pyrogallol[4]arenes were synthesized according to literature procedures. PgC2 and PgC4 were synthesized using the method reported by Gerkensmeier et al. modified by using acetaldehyde and valeraldehyde, respectively.11 Synthesis of 3,3′-(Propane-1,3-diyl)bis(1-methyl-1H-imidazol-3-ium) Dibromide {[C3(C1Im)2]Br2}. The preparation and cleanup of [C3(C1Im)2]Br2 were conducted following methods and precautions reported recently,12 yielding an analytically pure (>99.9%) material on the basis of 1H NMR and ESI-MS characterization. The resulting bromide salt was then twice recrystallized from acetonitrile and dried in vacuo before the studies described here were conducted. Crystallography. Data for cocrystals were collected using synchrotron radiation (λ = 0.7749 Å) with a Bruker Apex II CCD diffractometer. See the following paragraphs for further crystallographic information. Cocrystal 1. C83H106O27Br2N4; M = 1751.53; colorless plate; a = 34.430(3) Å; b = 10.1777(10) Å; c = 22.398(2) Å; β = 92.226(2)°; space group C2/c; V = 7842.7(12) Å3; Z = 4; Dc = 1.48 g/cm3; F000 = 3672; synchrotron radiation; λ = 0.7749 Å; T = 100 K; 6936 reflections collected; final goodness of fit = 1.071; R1 = 0.058; wR2 = 0.148; R indices based on reflections with I (refinement on F2); 615 parameters; no restraints; Lp and absorption corrections applied; μ = 1.379 mm−1. Cocrystal 2. C99H144O31Br2N4; M = 2046; colorless block; a = 44.5094(15) Å; b = 12.9169(4) Å; c = 17.9977(6) Å; β = 108.483(2)°; space group C2/c; V = 9813.6(6) Å3; Z = 4; Dc = 1.39 g/cm3; F000 = 4336; synchrotron radiation; λ = 0.7749 Å; T = 100 K; 6021 reflections collected; final goodness of fit = 1.041; R1 = 0.088; wR2 = 0.24; R indices based on reflections with I (refinement on F2); 699 parameters; 30 restraints; Lp and absorption corrections applied; μ = 1.119 mm−1. Cocrystal 3. C151.95H223.40O46.75Br3N6; M = 3121.88; colorless prism; a = 18.4096(6) Å; b = 20.1880(6) Å; c = 23.4074(7) Å; α = 73.340(2)°; β = 68.043(2)°; γ = 79.237(2)°; space group P1̅; V = 7698.7(4) Å3; Z = 2; Dc = 1.35 g/cm3; F000 = 3312; synchrotron radiation; λ = 0.77490 Å; T = 100 K; 20950 reflections collected; final goodness of fit = 1.528; R1 = 0.117; wR2 = 0.348; R indices based on reflections with I (refinement on F2); 2136 parameters; 417 restraints; Lp and absorption corrections applied; μ = 1.071 mm−1.

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CONCLUSIONS This work offers an expansion of the supramolecular chemistry for alkyl-tethered geminal ionic liquid dications with Calkylpyrogallol[4]arene hosts. This report nicely illustrates how the solvent can steer the solid-state structure in ways difficult to predict and also evidently dependent upon the alkyl chain pendant to the pyrogallol[4]arene subunits. In particular, the structure of a dimeric host−guest complex consisting of PgC2 and [C3(C1Im)2]Br2 is isostructural regardless of whether the complex crystallized from methanol or from acetonitrile. Crystallization of the butyl pyrogallol[4]arene macrocycle PgC4 with [C3(C1Im)2]Br2 in a 9:1 acetone/water mixture generates “Pac-man”-shaped dimeric host−guest capsules assembled into bilayer galleries. When the solvent system is changed to methanol, a bilayer-type arrangement is again realized but with a marked difference. In this case, the formation of two distinct dimeric host−guest assemblies is apparent: a dimeric capsule and an offset dimer. Additionally, in cocrystals 2 and 3, the alkyl chains of the pyrogallol[4]arenes are arranged in two different motifs, one ordered (cocrystal 2) and the second not ordered (cocrystal 3). The alkyl chain may be playing a part in the structural packing, and further studies involving longer alkyl chains will be pursued. Given the practically unlimited molecular flexibility and diverse functionality available to ionic liquids, we believe this will open up entirely new perspectives in crystal engineering. For example, the crystallization of bitethered tricationic species has not yet been realized, although this should certainly witness a remarkable multiplicity of fascinating cocrystal forms upon supramolecular assembly with pyrogallol[4]arenes. Studies along these lines of inquiry are currently under way in our laboratories and will be reported in due course.

S Supporting Information *

Single-crystal X-ray crystallographic information files (CIF) are available for all cocrystals. Crystallographic information files are also available from the Cambridge Crystallographic Data Center (CCDC) upon request (http://www.ccdc.cam.ac.uk, CCDC deposition numbers 1004310−1004312). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: (573) 882-1811. *E-mail: [email protected]. Telephone: (573) 882-8374. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.L.A. thanks the National Science Foundation for funding. G.A.B. acknowledges start-up funding from the University of MissouriColumbia used to support this work. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. 4203

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ABBREVIATIONS [C3(C1Im)2]Br2, 3,3′-(propane-1,3-diyl)bis(1-methyl-1H-imidazol-3-ium) dibromide; PgC2, C-ethylpyrogallol[4]arene; PgC4, C-butylpyrogallol[4]arene

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REFERENCES

(1) (a) Busi, M.; Laurenti, M.; Condorelli, G. G.; Motta, A.; Favazza, M.; Fragalà, I. L.; Montalti, M.; Prodi, L.; Dalcanale, E. Chem.Eur. J. 2007, 13, 6891−6898. (b) Pinalli, R.; Cristini, V.; Sottili, V.; Geremia, S.; Campagnolo, M.; Caneschi, A.; Dalcanale, E. J. Am. Chem. Soc. 2004, 126, 6516−6517. (2) (a) Dalgarno, S. J.; Cave, G. W. V.; Atwood, J. L. Angew. Chem., Int. Ed. 2006, 45, 570−574. (b) Kulikov, O. V.; Daschbach, M. M.; Yamnitz, C. R.; Rath, N.; Gokel, G. W. Chem. Commun. 2009, 7497− 7499. (3) (a) Dalgarno, S. J.; Power, N. P.; Warren, J. E.; Atwood, J. L. Chem. Commun. 2008, 1539−1541. (b) Dalgarno, S. J.; Power, N. P.; Atwood, J. L. Coord. Chem. Rev. 2008, 252, 825−841. (c) Dalgarno, S. J.; Power, N. P.; Warren, J. E.; Atwood, J. L. Chem. Commun. 2008, 1539. (d) Jin, P.; Dalgarno, S. J.; Atwood, J. L. Coord. Chem. Rev. 2010, 254, 1760−1768. (e) Jin, P.; Dalgarno, S. J.; Warren, J. E.; Teat, S. J.; Atwood, J. L. Chem. Commun. 2009, 3348−3350. (f) McKinlay, R. M.; Cave, G. W. V.; Atwood, J. L. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5944−5948. (4) (a) Mossine, A. V.; Kumari, H.; Fowler, D. A.; Maerz, A. K.; Kline, S. R.; Barnes, C. L.; Atwood, J. L. Isr. J. Chem. 2011, 51, 840− 842. (b) Mossine, A. V.; Kumari, H.; Fowler, D. A.; Shih, A.; Kline, S. R.; Barnes, C. L.; Atwood, J. L. Chem.Eur. J. 2012, 18, 10258− 10260. (5) (a) Kumari, H.; Zhang, J.; Erra, L.; Barbour, L. J.; Deakyne, C. A.; Atwood, J. L. CrystEngComm 2013, 15, 4045−4048. (b) Fowler, D. A.; Tian, J.; Barnes, C.; Teat, S. J.; Atwood, J. L. CrystEngComm 2011, 13, 1446−1449. (c) Fowler, D. A.; Tian, J.; Barnes, C.; Teat, S. J.; Atwood, J. L. CrystEngComm 2011, 13, 1446. (6) (a) Dalgarno, S. J.; Bassil, D. B.; Tucker, S. A.; Atwood, J. L. Angew. Chem., Int. Ed. 2006, 45, 7019−7022. (b) Dalgarno, S. J.; Tucker, S. A.; Bassil, D. B.; Atwood, J. L. Science 2005, 309 (5743), 2037−2039. (c) Kline, K. K.; Fowler, D. A.; Tucker, S. A.; Atwood, J. L. Chem.Eur. J. 2011, 17, 10848−10851. (7) (a) H. Davis, J. J. Chem. Lett. 2004, 33, 1072−1077. (b) Tang, S.; Baker, G. A.; Zhao, H. Chem. Soc. Rev. 2012, 41, 4030−4066. (8) (a) Abbott, A. P.; Frisch, G.; Ryder, K. S. Annu. Rev. Mater. Res. 2013, 43 (1), 335−358. (b) Ho, T. D.; Zhang, C.; Hantao, L. W.; Anderson, J. L. Anal. Chem. 2013, 86 (1), 262−285. (c) MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H.; Watanabe, M.; Simon, P.; Angell, C. A. Energy Environ. Sci. 2014, 7 (1), 232−250. (d) Pusey, M. L.; Paley, M. S.; Turner, M. B.; Rogers, R. D. Cryst. Growth Des. 2007, 7 (4), 787− 793. (9) (a) Fowler, D. A.; Atwood, J. L.; Baker, G. A. Chem. Commun. 2013, 49 (18), 1802−1804. (b) Fowler, D. A.; Teat, S. J.; Baker, G. A.; Atwood, J. L. Chem. Commun. 2012, 48 (43), 5262−5264. (c) Fowler, D. A.; Pfeiffer, C. R.; Teat, S. J.; Beavers, C. M.; Baker, G. A.; Atwood, J. L. CrystEngComm 2014, in press. (10) (a) Ling, I.; Alias, Y.; Raston, C. L. New J. Chem. 2011, 35 (7), 1549−1555. (b) Ling, I.; Alias, Y.; Skelton, B. W.; Raston, C. L. Aust. J. Chem. 2010, 63 (10), 1492−1496. (c) Ling, I.; Alias, Y.; Skelton, B. W.; Raston, C. L. Dalton Trans. 2012, 41 (16), 4884−4889. (d) Ling, I.; Alias, Y.; Sobolev, A. N.; Raston, C. L. Cryst. Growth Des. 2009, 9 (10), 4497−4503. (e) Ling, I.; Alias, Y.; Sobolev, A. N.; Raston, C. L. CrystEngComm 2010, 12 (2), 573−578. (11) Gerkensmeier, T.; Iwanek, W.; Agena, C.; Fröhlich, R.; Kotila, S.; Näther, C.; Mattay, J. Eur. J. Org. Chem. 1999, 1999 (9), 2257− 2262. (12) (a) Burrell, A. K.; Del Sesto, R. E.; Baker, S. N.; McCleskey, T. M.; Baker, G. A. Green Chem. 2007, 9, 449−454. (b) Mutelet, F.; Hassan, E.-S. R. E.; Stephens, T. W.; Acree, W. E.; Baker, G. A. J. Chem. Eng. Data 2013, 58, 2210−2218. 4204

dx.doi.org/10.1021/cg500793z | Cryst. Growth Des. 2014, 14, 4199−4204