Constructing Multicomponent Materials Involving Inclusion of Mono

Mar 21, 2013 - Synopsis. p-Sulfonatocalix[5]arene forms host−guest complexes in the solid state with 3-(2-hydroxyethyl)-1-methylimidazolium and 1,1â...
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Constructing Multicomponent Materials Involving Inclusion of Mono- and Bis-Imidazolium Cations in Gadolinium(III)‑p‑sulfonatocalix[5]arene Coordination Networks Irene Ling,*,† Ramiz A. Boulos,‡ Brian W. Skelton,§ Alexandre N. Sobolev,§,∥ Yatimah Alias,† and Colin L. Raston*,‡,⊥ †

Chemistry Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Centre for Strategic Nano-Fabrication, School of Chemistry and Biochemistry, M310, The University of Western Australia, 35 Stirling Highway, Crawley, West Australia 6009, Australia § Centre for Microscopy, Characterisation and Analysis, M010, The University of Western Australia, Perth, West Australia 6009, Australia ∥ School of Chemistry and Biochemistry, M310, The University of Western Australia, 35 Stirling Highway, Crawley, West Australia 6009, Australia ⊥ School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia 5042, Australia ‡

S Supporting Information *

ABSTRACT: p-Sulfonatocalix[5]arene forms host−guest complexes in the solid state with hydroxyl-functionalized imidazolium and bis-imidazolium cations, also involving gadolinium(III) ions which form a coordination polymer through the sulfonate groups of the calixarene. The interplay of the components have been investigated using Hirshfeld surface analysis, with the nature of the species in aqueous solution in the absence of the paramagnetic lanthanide ions studied using 1H NMR and modeled using molecular simulations.

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other calix[n]arenes (n = 4, 6, and 8) with the same groups, notably in forming an amorphous solid.3 p-Sulfonatocalix[5]arene usually adopts the cone conformation in the solid state with hydrogen-bonding between phenolic oxygen atoms on the lower rim.4 Other conformations include the pseudo double cone and the partial cone.5 Importantly this calixarene can assemble into bilayers, analogous to the clay-like arrangement of the smaller calix[4]arene.6 A dense packed bilayer arrangement of p-sulfonatocalix[5]arene forms in the presence of ytterbium(III), sodium, and tetraphenylphosphonium ions, with the latter acting only as an auxiliary in facilitating the crystallization of the metal-calixarene complex.7 In addition, p-sulfonatocalix[5]arene affords molecular capsule arrangements based on two calixarenes from adjacent bilayers, either in the presence of aquated lanthanide ions (Eu3+, Tb3+, Gd 3+, and Yb3+) or pyridine type molecules such as

alix[n]arenes are one of the most studied organic cyclic oligomers with versatility in forming host−guest complexes depending on the number of phenol moieties and the nature of the upper and lower rim substituents. The inclusion and supramolecular chemistry of water-soluble calix[n]arenes with sulfonate groups on the upper rim have been extensively studied, mostly for the calix[4]arene as the smallest macrocycle in the series.1 In contrast, the inclusion and supramolecular chemistry of the corresponding calix[5]arene are less developed because of difficulties in gaining access to significant quantities of the parent precursor calixarene bearing tert-butyl groups on the upper rim, and its higher conformational mobility relative to calix[4]arene.2 The 5-fold symmetry of the molecule in the common symmetrical cone conformation is interesting, particularly in the packing of the macrocycle in regular arrays, at the same time with the larger cavity having potential for binding larger molecules relative to the smaller p-sulfonatocalix[4]arene. In this context, we recently established that calix[5]arene bearing phosphonic acid groups located at the para-positions of the cavitand is distinctly different from the © 2013 American Chemical Society

Received: January 17, 2013 Revised: March 12, 2013 Published: March 21, 2013 2025

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Figure 1. Host−guest complexation involving p-sulfonatocalix[5]arene and imidazolium cations.

Figure 2. Cartoon showing the coordination polymer arrangement and hydrophilic channels in the extended structure in complex 1.

phenanthroline and bipyridinium compounds.5b,8 The sulfonic acid analogue of the calix[5]arene forms dimeric capsules with a sulfuric acid molecule residing in each calixarene cavity.9 In other studies, complicated motifs of the calixarene have also been established, for example, the Ferris wheel-like coordination polymers involving complexation of the host calixarene with aza-crown ethers and lanthanide ions.10 We are interested in the complexation of water-soluble psulfonatocalix[n]arenes with imidazolium cations of varying size and functionality, along with phosphonium and lanthanide cations. Multicomponent crystalline material built of psulfonatocalix[4]arene bilayers with the imidazolium cations selectively drawn into the calixarene cavity have the metal interacting with sulfonate groups in the water-rich regions and the phosphoniums embedded within the hydrophobic bilayer. Recently, we reported an intermolecular complexation of watersoluble p-sulfonatocalix[5]arene with large organic bis-

phosphonium cations where the interplay of the cation can be both exo- and endo- relative to the cavity of the calixarene in a distorted cone conformer devoid of imidazolium molecules in the cavity.11 In further understanding the host−guest chemistry of p-sulfonatocalix[5]arene and imidazolium based cations, we have investigated the interplay of a hydroxy functionalized imidazolium cation and a bis-imidazolium cation, replacing the simple monoalkyl-imidazolium cations in the solid state, Figure 1, along with solution studies and molecular modeling. We have found that the supermolecules built of 1,1′-[1,4-phenylenebis(methylene)]bis(3-butyl-1H-imidazolium-1-yl) or 3-(2-hydroxyethyl)-1-methylimidazolium and p-sulfonatocalix[5]arene show charge distribution complementarity with higher stability relative to the monoalkyl-imidazolium with p-sulfonatocalix[5]arene systems. A number of attempts were made to form crystalline complexes of p-sulfonatocalix[5]arene with lanthanide ions and phosphonium cations in the presence of the 12026

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Figure 3. Projection of complex 1 down the c-axis showing the extended structure, water molecules, and gadolinium(III) ions have been removed for ease of visualization.

Figure 4. Ball and stick representation of Gd(1) coordinated to four different calixarene sulfonate groups in complex 1.

structures described herein, complexes 1 and 2. Symmetry expansion of the asymmetric unit for both complexes shows that the extended network contains a bilayer arrangement, with the calixarenes aligned from adjacent bilayers as skewed molecular capsules confining imidazolium cations. For complex 1 one gadolinium(III) metal center coordinates to four independent calixarenes, whereas for complex 2 one-halfpopulated disordered gadolinium(III) center coordinates to one calixarene with the neighboring calixarenes involved in secondary coordination sphere interactions, with an independent disordered gadolinium(III) ion (site occupancy of 0.25) interacting with two calixarenes with the sulfonate groups facing each other and involved in the secondary coordination sphere interactions. Complex 1 has an open channel structure

alkyl-3-imidazolium ions which has the size and shape to bind in the cavity of the calixarene, but they always resulted in amorphous material. This may arise from the inability to pack 5-fold symmetry molecules into flat sheets in an ordered array. Interestingly, the iso-electronic p-phosphonatocalix[5]arene is always isolated as amorphous material, forming lower rim to upper rim stacked fibers from solutions containing molecular capsules based on two calixarenes being held together by a seam of hydrogen bonds.3

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RESULTS AND DISCUSSION

Colorless crystalline material was isolated and characterized using single crystal X-ray diffraction, in establishing the two 2027

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Figure 5. Fingerprint plots for p-sulfonatocalix[5]arene (a) and 1-(2-hydroxyethyl)-3 methylimidazolium (b) in complex 1.

the hydrogen-bonding appears as two symmetrical spikes (circled in red), Figure 5. Gd1 atoms are sited on 2-fold axes and are eight-coordinate with the coordination spheres consisting of four oxygen atoms from four calixarene sulfonate groups and four water molecules (Gd-OSO2 contacts at 2.390(5)−2.394(5) Å). Coordination of four sulfonate groups to a lanthanide center is unusual with the bilayers usually separated by a hydrophilic region, and the lanthanide ions are unable to coordinate to sulfonate groups from adjacent bilayers, where coordination with one or two or indeed no coordination to sulfonate ions can occur (see below for Gd). A similar coordination environment has been reported for the smaller analogous calix[4]arene where one metal center (Ce3+, Nd3+, Sm3+, or Eu3+) is bound to four sulfonate groups from four neighboring p-sulfonatocalix[4]arenes.13 Fully aquated Gd(2) atoms are disordered over two positions in the channels (site occupancy of 0.25) and are involved in secondary coordination interactions with the calixarene sulfonate group and included water molecules, the closest GdO···OSO2 distances being 3.92 Å and GdO···O(water) being 2.70 to 3.33 Å. The 3-(2-hydroxyethyl)-1-methylimidazolium lies in the calixarene cavity, which is distorted from 5-fold symmetry (see Figure 6). The dihedral angles between the plane through the methylene carbon atoms and the phenyl ring planes are 76.1(2), 39.6(2), 79.0(2), 31.0(3), and 54.3(3)° for rings 1−5 respectively. The oxygen atom of the hydroxyl group of the imidazolium cation has a close contact to the sulfonate oxygen atom of the centrosymmetrically related calixarene with an O···O distance of 2.69 Å. The aforementioned interaction is apparent in the fingerprint plot as a red streak which highlights the important contribution of the O···O contact, Figure 5. Although water molecule hydrogen atoms were not located, the O···O distances suggest that the coordinated water molecules are also involved in hydrogen bonds to the hydroxyl oxygen atom and also to sulfonate oxygen atoms, the O···O distances being 2.79 Å and 2.70−2.82 Å respectively. Other interactions involving the imidazolium and calixarene include (i) H-bonding with C−H···O−S distances at 2.43−2.86 Å (circled in red) as a dominant contribution to the overall intermolecular interactions, making up 48% of the Hirshfeld surface, and (ii) C− H···π interactions with C···π distances at 3.54−3.77 Å (circled in pink) contributing 22% to the Hirshfeld surface. The imidazolium cation fits snugly into the calixarene cavity, with the five-membered ring being positioned approximately

with polymeric coordination chains and included functionalized imidazolium ions in the calixarene cavity, along with hydrophilic regions occupied by a large quantity of water molecules and aquated lanthanide ions. In complex 2, bis-imidazolium cations reside between the bilayers, as for the cations in complex 1, being in the skewed “molecular capsules” and adjacent to them. The overall the structure of complex 2 is more compact than that of complex 1. Complex 1: 3-(2-Hydroxyethyl)-1-methylimidazolium in p-sulfonatocalix[5]arene. Addition of 3-(2-hydroxyethyl)1-methylimidazolium to p-sulfonatocalix[5]arene in the presence of gadolinium(III) ion resulted in the formation of colorless plates of the gadolinium bridged calixarene complex 1, with the imidazolium residing in the cavity. The structure crystallizes in the monoclinic space group C2/c, z = 4, with pairs of centrosymmetrically related calixarenes coordinated by gadolinium(III) atoms through the oxygen atoms of sulfonate groups. Further analysis of the extended solid state structure shows that polymeric chains of calixarenes are associated with channels (viewing down the c-axis) occupied by aquated gadolinium(III) ions and water molecules. These are involved in extensive hydrogen bonding, evident from the close proximity of the metal ions and water molecules to the sulfonate groups and within themselves, Figure 2. The hydrated channels are parallel to the c-axis, ca. 10 × 10 Å2 in cross section, in a way similar to that recently reported by Su et al. for an inclusion complex involving 1,2-bis(4,4′-methylpyridinium)ethane and p-sulfonatocalix[5]arene.6b The extended structure takes on a bilayer arrangement, Figure 3, with the bilayer thickness ca. 11 Å, with each layer connected through lanthanide-calixarene coordination interactions which affords the polymeric array. Four calixarenes are coordinated to one gadolinium(III) ion in a square antiprism geometry, and form skewed “molecular capsules” ca. 15 Å in height, with Gd−OSO2 distances at 2.390(4)−2.415(5) Å, Figure 4. Intermolecular C−H···π interaction and hydrogenbonding between the capsules are evident by close C−H···π distance at 2.69 Å (corresponding C···π distance at 3.39 Å) and C−H···H−C distances from 2.50 to 2.86 Å (corresponding C···C distance at 2.91 to 3.86 Å) respectively. Both interactions are revealed in the fingerprint plot, generated from the Hirshfeld surface analysis12 where wings (circled in pink) at the upper left and bottom right of the plot correspond to the C−H···π (donor) and C−H···π (acceptor) interactions while 2028

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coordinated calixarene molecules in the ‘disordered’ form is complicated, and we tried to present the image of the ‘proper’ positioning of the calixarene molecules in neighbors crystal unit cells with ‘disordered’ components removed as depicted in Figure 7. The calixarene molecules distorted from the ideal 5fold symmetrical cone conformation are arranged in skewed molecular capsules. This is evident on examining the angles between the least-squares planes of the methylene carbon atoms and the phenolic rings, the dihedral angles being 68.6(2), 34.4(2), 80.0(2), 64.9(2), and 32.6(2)° for rings 1−5 respectively. We have established a similar interplay of the bis-imidazolium cation through end-capping by two calixarene molecules for the smaller cone conformer p-sulfonatocalix[4]arene, in affording analogous molecular capsules.14 The extended structure of complex 2 has a comparable bilayer arrangement to that of complex 1, with the “skewed molecular capsules” confining cis-bis-imidazolium cations, and with interactions with disordered gadolinium(III) metal centers either as primary or secondary coordination interactions. Gd(1) coordinates to one calixarene sulfonate group with Gd−OSO2 distance at 2.415(9) Å, with the metal ions coordinated directly by seven water molecules; the associated Gd−O(water) distances range from 2.383(17) to 2.510(4) Å), Figure 7, with two nearest neighboring calixarenes forming secondary coordination interaction to the metal ion with Gd−O···O distances ranging from 2.65 to 2.90 Å. In our case, the “disorder” is a result of superposition in one asymmetric unit the calixarene molecules with differently positioned Gd(1) cations in neighbor units. In reality, the gadolinium(III) cation is bounded only with one calixarene molecule. There are water molecules involved in extensive H-bonding with the O-atom containing fragments of the calixarene phenolic and sulfonate groups and aquated gadolinium ions, as well as with other water molecules, with O···O distances in the range 2.45(3)−2.94(4)

Figure 6. Stick representation of the close intermolecular contacts of the imidazolium cation and calixarene in complex 1, green dotted lines for C−H···O contacts, red dotted line for close O···O contacts, blue dotted lines for C−H···π interactions, and black dotted line for π···π interactions.

parallel to one of the calixarene phenolic rings with the π···π centroid to centroid distance at 3.77 Å. A similar arrangement involves p-sulfonatocalix[4]arene with the π···π centroid to centroid distance at 3.75 Å.12 Complex 2: 1,1′-[1,4-Phenylenebis(methylene)]bis(3methylimidazolium-1-yl) in p-sulfonatocalix[5]arene. Complex 2 crystallizes in the space group P1̅, Z = 1, comprising two p-sulfonatocalix[5]arenes in the asymmetric unit with their upper rims facing each other, sharing one bis-imidazolium with the alkyl terminus residing in the cavities of the calixarenes. The overall structure for the ratio 3:1 of noncoordinated and Gd-

Figure 7. A ball and stick representation of part of the structure of complex 2 showing the skewed molecular capsule arrangement and metal coordination environments. 2029

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Figure 8. Fingerprint plots for 1-(2-hydroxyethyl)-3-methylimidazolium (a) and p-sulfonatocalix[5]arene (b) for complex 2.

Figure 9. Projection of complex 2 down the c-axis showing the extended structure.

2.51 to 2.99 Å (corresponding C···O distance 3.45 and 3.81 Å); (ii) C−H···π(centroid) interaction from the aromatic protons to the calixarene aromatic rings with close distances at 2.71 and 2.85 Å (corresponding C···π distance at 3.65 and 3.61 Å); and (iii) π···π centroid to centroid distance at 3.80 and 3.97 Å and where the imidazolium aromatic ring is positioned approximately parallel to one of the calixarene phenolic rings. These interactions are shown in the fingerprint plot as yellow and pink circles that correspond to C−H···O and C−H···π interactions respectively, Figure 8. The pores within the extended structure are lost relative to complex 1, in replacing the guest molecule with a bisimidazolium cation, Figure 9. The organization of the supermolecules (effectively molecular capsules) relates to the hydrophobic interplay involving the π-stacking between adjacent calixarenes with the aromatic π(centroid)···π(centroid) distance at 3.82 Å, which is also evident from the fingerprint plot (black circle). There is no evidence of C−H···π interactions involving the methylene bridges within the calixarenes to aromatic rings. Other close contacts involve Hbonding between neighboring calixarenes with C−H···O distances at 2.47−2.94 Å (corresponding C···O distance 3.29 and 3.90 Å), and they contribute 35% to the overall interactions (yellow circle). There are two independent bis-imidazolium

Å. Gd(2) and Gd(3) are disordered and have been modeled with site occupancies of 0.25 as eight-coordinate with one or more coordinated water molecules not located (Gd−O distances ranging from 2.251(9) Å to 2.303(10) Å). They have secondary coordination interactions to two calixarenes with their upper rims facing each as skewed ‘molecular capsules’ with the Gd−O···O distance of 2.46−2.92 Å. The centrosymmetric bis-imidazolium cation is in a cisconfiguration relative to the central aromatic ring and is endcapped by two calixarene molecules (capsule size being ca. 16 Å), with the terminus having a similar interplay within the calixarene cavities. However, they differ significantly in the orientation of the five-member rings such that one dication has a methyl group directed toward aromatic rings, with the C− H···π(centroid) distance at 2.69 and 3.05 Å (corresponding C···π distance of 3.59 and 3.51 Å respectively), whereas the other dication has a methyl group directed toward the sulfonate groups, with the C···O distance at 4.42 Å. Both interactions are depicted in the fingerprint plot as pink and yellow circles respectively with corresponding contributions of 22% and 44% to the Hirshfeld surface. The complementarity of interaction of the methyl imidazolium terminus with the calixarene include (i) weak H-bonding between the bis-imidazolium and the calixarene upper rim, with short C−H···O distances, from 2030

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Figure 10. Stick projection (along the c-axis) of complex 2 showing the π···π interaction (black dotted lines) between the bis-imidazoliums and calixarene (endo-cis-dication in dark blue, exo-trans-dication in light blue); gadolinium(III) ions have been removed for ease of visualization.

Figure 11. Ratio of liquid phase energy per atom on solid phase energy per atom for the three systems of p-sulfonatocalix[5]arene and imidazoliums. CPK representation of the final geometry of the imidazolium molecules in the pocket of p-sulfonatocalix[5]arene after molecular dynamics. From left to right 1,1′-[1,4-phenylenebis(methylene)]bis(3-butyl-1H-imidazolium-1-yl), 1-(2-hydroxyethyl)-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-octyl-3-methylimidazolium, and 1-butyl-3- methylimidazolium. The p-sulfonatocalix[5]arene molecules are in green and the imidazolium molecules are in brown. N atoms are in blue, O in red, and S in yellow.

cations in trans-configurations located exo relative to the molecular capsules and are involved in π···π interactions to the encapsulated cis-dication and calixarene. One of the trans-bisimidazolium ions has its central benzene ring in close contact with a calixarene phenolic ring with the π(centroid)···π(centroid) distance at 4.02 Å, while the other trans-bisimidazolium has both its imidazolium moieties interacting with the central benzene rings of the included cis-bis-imidazolium cation with π(centroid)···π(centroid) distances of 3.40 and 3.79 Å, Figure 10. The overall arrangement of calixarenes in the

bilayers is approximately 12 Å thick, and the inter bilayer is approximately 17 Å thick.

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MOLECULAR MODELING The formation of a stable supermolecule on the basis of two species in aqueous solution, which ultimately yields single crystals, was studied using molecular dynamics calculations. Ideally, the attraction between the two species is greater than the solvation energy of the species, and the free energy of crystallization is greater than the free energy of the solvated adduct. This depends on the energy contour of the two species 2031

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as they coalesce to produce the supermolecules and the probabilistic nature of the energy well and usually involves calculating the free energy of the supermolecule from a solid phase into a liquid phase or vice versa. Here we use a simplified approach of calculating and comparing the ratios of the energies of solvated p-sulfonatocalix[5]arene with different imidazoliums, in this case 1-(2-hydroxyethyl)-3-methylimidazolium, 1butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1octyl-3-methylimidazolium, and 1,1′-[1,4-phenylenebis(methylene)]bis(3-butyl-1H-imidazolium-1-yl). The energy of the supermolecules in the solid phase was derived from minimized energy calculations using the crystal structure of complex 1 as the basis, and the liquid phase energy was calculated after applying a molecular dynamics calculation of the system for a sufficient time for it to equilibrate. Similar calculations were carried out for other systems, however, using different alkyl-imidazolium molecules (1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-octyl-3-methylimidazolium, and 1,1′-[1,4-phenylenebis(methylene)]bis(3-butyl-1Himidazolium-1-yl) replacing 1-(2-hydroxyethyl)-3-methylimidazolium, and in doing so, the total number of atoms in the system has been modified. In comparing the ratio of energies irrespective of the size of the system, the energy values were presented as the energy/atom. The ratio of the liquid phase energy per atom on the solid phase energy per atom, Figure 11, shows a decline as 1-(2-hydroxyethyl)-3-methylimidazolium was replaced with 1-hexyl-3-methylimidazolium, and a further decline as it was replaced with 1-octyl-3-methylimidazolium and 1-butyl-3-methylimidazolium. These suggest that the latter systems are less stable. The information gained from the molecular dynamics also illustrates that the hydrophobic tails of the imidazolium molecules do not point into the cavity of the psulfonatocalix[5]arene to minimize interactions with the solvent surrounding it, Figure 11. This is not surprising since the lower rim of the p-sulfonatocalix[5]arene has five hydroxyl groups and is thus highly polar. When the functionalized imidazolium was substituted with bis-imidazolium, the ratio of the energy of the phases increased sharply suggesting that the supermolecule is more stable compared to the functionalized imidazolium with p-sulfonatocalix[5]arene. The charge distributions for all species were calculated to further explain why the optimum geometry structures of some supermolecules might produce crystals while others might not, Figure 12. The negative charge on the 1-(2-hydroxyethyl)-3methylimidazolium is relatively concentrated at the oxygen atom which complements the charge distribution on the psulfonatocalix[5]arene. However, for 1-butyl-3-methylimidazolium, the negative charge is localized, while for 1-hexyl-3methylimidazolium and 1-octyl-3-methylimidazolium, the negative charge is spread along the hydrophobic alkyl tails of the imidazoliums which results in an unfavorable interaction. In the case of p-sulfonatocalix[5]arene and 1-(2-hydroxyethyl)-3methylimidazolium, there is strong attraction between the polar hydroxyl group of the 1-(2-hydroxyethyl)-3-methylimidazolium molecule and the polar sulfonato moiety of the psulfonatocalix[5]arene. However, this is not the case when 1(2-hydroxyethyl)-3-methylimidazolium is replaced with the alkyl imidazoliums where the hydrophobic alkyl chain results in a hydrophobic mismatch in the calixarene system. The bisimidazolium has a similar charge distribution profile around the imidazolium group to the other imidazoliums without the negative charge distribution in the rest of the molecule which would have otherwise resulted in an unfavorable interaction due

Figure 12. Minimized structures of the p-sulfonatocalix[5]arene (a) and the imidazoliums (b−f) (on the left in ball and stick representation) predicted by Spartan using the AM1 Hamiltonian. On the right are the electrostatic potential surfaces mapped onto the electron density maps of the structures. H atoms are colored in white, N in blue, O in red, and S in yellow.

to the negative charge distribution on the sulfonato moiety of the calixarene.

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SOLUTION STUDIES H NMR solution studies were undertaken to establish the nature of the interplay of the two ions (p-sulfonatocalix[5]arene and 3-(2-hydroxyethyl)-1-methylimidazolium or 1,1′-[1,4phenylenebis(methylene)]bis(3-butyl-1H-imidazolium-1-yl), in the absence of paramagnetic lanthanide ions, for 2:1 and 1:1 mixture of the two species at 25 °C (D2O), Figure 13. Host− guest formation is evident for the hydroxyl functionalized imidazolium cation, in accordance with the solid state structure with upfield chemical shifts experienced by all the protons of the imidazolium cation. The aromatic protons have the largest shift, which is consistent with the shielding effect of the 1

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Figure 13. 1H NMR spectra for 3-(2-hydroxyethyl)-1-methylimidazolium cation (a); p-sulfonatocalix[4]arene with 3-(2-hydroxyethyl)-1methylimidazolium (1:1) (b) and (2:1) (c); 1,1′-[1,4-phenylenebis(methylene)]bis(3-butyl-1H-imidazolium-1-yl) (d); p-sulfonatocalix[4]arene with 1,1′-[1,4-phenylenebis(methylene)]bis(3-butyl-1H-imidazolium-1-yl) (1:1) (e) and (2:1) (f), measured in D2O, (*) as solvent residues and impurities.

yethyl)-1-methylimidazolium in the cavity of the bowl-shaped p-sulfonatocalix[5]arene. Here, the functional group of the cation plays an important role in the association of the imidazolium cation with the calixarene, and this brings a new design strategy in constructing (imidazolium-p-sulfonatocalix[5]arene) supermolecules in building complex bilayers. The nature and the complexity of interaction of the supermolecule in the solid state structure are understood through Hirshfeld surface mapping, and molecular simulations, with 1H NMR establishing that the functional group of the imidazolium cation is essential in restricting conformational mobility of the calixarene.

calixarene with the imidazolium cation in the cavity of the calixarene. There is a significant difference in the conformation of the macrocylic when different ratios of hydroxyl functionalized imidazolium cation were added, apparent from the methylene protons at the 3.5−4.5 ppm region, Figure 13. At 1 equiv. of calix[5]arene to 1 equiv. of imidazolium, the methylene protons appeared as a sharp peak, whereas a lower and less sharp peak was seen when 2 equiv. amount of imidazolium was added to 1 equiv. of calix[5]arene where this presumably arises from the exchange on the NMR time scale along with the conformational flexibility of p-sulfonatocalix[5]arene in solution. The 1H NMR data also establish the persistence of molecular capsules in solution when the psulfonatocalix[5]arene/bis-imidazolium (2:1) complex solution was studied, with significant appearance of the calixarene methylene protons at 3.5−4.0 ppm that corresponds to the cone conformation of the calixarene and also the slowing down of the exchange process on the NMR time scale, which can be related to the electrostatic and H-bonding interactions. Upfield chemical shift (∼1.2 ppm) experienced by the aromatic protons is consistent with the shielding of the imidazolium terminus associated with end-capping of the bis-imidazolium cation by the calixarenes. Overall, the persistence of the supermolecules in solution is noteworthy.

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EXPERIMENTAL SECTION

Synthesis of 1 and 2. p-Sulfonatocalix[5]arene,6a,15 was synthesized according to literature procedures. 1-(2-Hydroxyethyl)-3methylimidazolium trifluoromethanesulfonate and gadolinium(III) chloride hexahydrate salts were purchased from Sigma Aldrich and used as received. A hot solution of GdCl3·6H2O in water (0.030 M) was added to a hot solution of equimolar (0.010 M) p-sulfonatocalix[5]arene with 1-(2-hydroxyethyl)-3-methylimidazolium in a mixture of THF and water (2 mL). The solutions were left to cool and to evaporate slowly, with crystals forming after several days. The homogeneity of the materials was checked from the determination of cell dimensions of several crystals. X-ray Crystallography. Data were measured from single crystals using an Oxford Diffraction Gemini (for 1) and Oxford Diffraction Xcalibur (2) diffractometers at T = 100(2) K with monochromatic MoKα radiation (λ = 0.71073 Å). Following analytical (1) or multiscan (2) absorption corrections and solution by direct methods,

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CONCLUSIONS We have established a detailed understanding of self-assembly of a multiple component bilayer involving three different ions which has preferential binding of functionalized 3-(2-hydrox2033

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average energy derivative was less than 2 × 10−5 kcal/mol. Dynamic simulations were carried out using the Discover module and the NVT ensemble at 298 K with a time step of 1.0 fs as appropriate for a system in water. The total number of steps used for the simulation was 500 000 giving a total simulation time of 500.0 ps. Spartan 08 (Spartan Student Wave function, Inc. Irvine, CA) was used for calculating the electron densities of the structures in Figure 12. The energy of the structures was first minimized in the ground state using the semiempirical method and the AM1 Hamiltonian algorithm. The total charge assigned to the imidazoliums and the psulfonatocalix[5]arene was set to +1 (+2 for the dication species) and −5 respectively. The electron density surface was calculated for each of the structures with the potential property to produce the charge distribution on the structure.

the structures were refined against F2 with full-matrix least-squares using the SHELXL-9716 crystallographic package. Crystal Data for Complex 1. C82H115Gd1.42N4O60.50S10, M = 2668.68, colorless plates, 0.63 × 0.28 × 0.12 mm3, monoclinic, space group C2/c, a = 33.404(2), b = 19.1752(7), c = 22.1972(11) Å, = 122.090(8)°, V = 12045.6 Å3, Z = 4, Dc = 1.472 g/cm3, μ = 1.047 mm−1, 2θmax = 54.0°, 40734 reflections collected, 12603 unique (Rint = 0.0652). Final GOF = 1.002, R1 = 0.0798, wR2 = 0.2241, R indices based on 7769 reflections with I > 2σ(I), 800 parameters, 63 restraints. CCDC deposition number: 868751. One sulfonate group was modeled as being disordered over two sets of sites with occupancies refined to 0.724(13) and its complement. Geometries of the disordered components were restrained to ideal values. An area of relatively high electron density was modeled as a partially weighted Gd of a Gd(H2O)8 unit, disordered about a crystallographic 2-fold axis. Its site occupancy refined to 0.209(2). Coordinated water molecules were not located. The solvent atoms were modeled as water molecules with occupancies constrained to either 1.0 or 0.5 from refinement and geometrical considerations. The cations, presumably hydrogen ions, required for charge balance could not be located. Electron density that could not be reasonably modeled as such was effectively removed by use of the program Squeeze.14 Water molecule hydrogen atoms were not located. All remaining Hatoms were added at calculated positions and refined by use of a riding model with isotropic displacement parameters based on those of the parent atom. Anisotropic displacement parameters were employed for the non-hydrogen atoms. Crystal Data for Complex 2. C220H288Gd2N20O127S20, M = 6200.40, colorless plate, 0.26 × 0.22 × 0.13 mm3, triclinic, space group P1̅, a = 18.4693(5), b = 18.5141(5), c = 21.9625(6) Å, α = 104.299(2), β = 101.711(2), γ = 103.672(2)°, V = 6793.2(3) Å3, Z = 1, Dc = 1.516 g/ cm3, μ = 0.743 mm−1. 2θmax = 49.4°, 51953 reflections collected, 23143 unique (Rint = 0.0397). Final GOF = 1.192, R1 = 0.1665, wR2 = 0.3382, R indices based on 17105 reflections with I > 2σ(I) (refinement on F2), |Δρmax| = 4.6(2) e Å−3, 1538 parameters, 478 restraints. CCDC deposition number: 901317. One Gd atom was assigned occupancy of 0.5 with the atoms at the other two sites being assigned occupancies of 0.25 after trial refinement. The coordination spheres around these latter sites are not complete. One of the two p-sulfonatocalix[5]arene molecules was modeled as being disordered over two sets of sites of equal occupancy. The geometries of the disordered components were constrained to ideal values. The solvent was modeled as water molecules with partial occupancies. These disordered atoms were refined with isotropic displacement parameters. Anisotropic displacement parameters were employed for the remaining non-hydrogen atoms. Water molecule and phenol hydrogen atoms were not located. All remaining H-atoms were added at calculated positions and refined by use of a riding model with isotropic displacement parameters based on those of the parent atom. Charge balance requires protonation of one of the sulfonate groups of each of the calixarene molecules. Hirshfeld surface analysis was generated from the Crystal Explorer.12 Materials Studio (Accelrys. V5.5.3 2001−2012 Accelrys Software Inc.) was used for the molecular modeling. The crystal structure of 1(2-hydroxyethyl)-3-methylimidazolium and p-sulfonatocalix[5]arene was used as the basis of the molecular modeling. The Amorphous Cell module was used to construct a periodic cell with the crystal in a cell of water and the target density of the configuration set to 1 g/mL. Similarly, three periodic cells were produced where 1-butyl-3imidazolium, 1-hexyl-3-imidazolium, and 1-octyl-3-imidazolium replaced the 1-(2-hydroxyethyl)-3-methylimidazolium species leaving all other parameters unchanged. For the 1,1′-[1,4-phenylenebis(methylene)]bis(3-butyl-1H-imidazolium-1-yl), the crystal structure with p-sulfonatocalix[5]arene was used to produce a fourth periodic cell. Molecular species were assigned charges at physiological pH, and Na+ were added to counter balance the charges. For energy minimizations, the Discover module was used, employing the smart minimizer with ultrafine quality and the atom-based summation methods, and the PCFF Force Field. The convergence criterion of the

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(I.L.) Tel: (intl) +603 79676774. E-mail: [email protected]. edu.my. (C.L.R.) Tel: (intl) +61882017958. E-mail: colin. raston@flinders.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Malaya (HIR UM-MOHE F000004-21001), The University of Western Australia, and the Australian Research Council for supporting this work. The authors acknowledge the facilities and scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, funded by the University, State and Commonwealth Governments.

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REFERENCES

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