arene Binding with Large Bis-Phosphonium Cations - American

Article pubs.acs.org/crystal

Conformationally Flexible p-Sulfonated Calix[5]arene Binding with Large Bis-Phosphonium Cations Irene Ling,*,† Yatimah Alias,† Brian W. Skelton,‡ and Colin L. Raston*,§ †

Chemistry Department, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia Centre for Microscopy, Characterisation and Analysis, M313, University of Western Australia, Perth, Western Australia 6009, Australia § Centre for Strategic Nano-Fabrication, School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009 ‡

S Supporting Information *

ABSTRACT: Water-soluble p-sulfonated calix[5]arene forms a multicomponent complex with aquated Gd(III) ions and bisphosphonium cations, where the calixarenes are arranged in bilayers with the bis-phosphonium cations embedded between and within the bilayers. The calixarenes are significantly distorted from the common “cone” conformation and are linked from one bilayer to another through the sulfonate groups via secondary coordination sphere interactions associated with homoleptic aquated metal ions.

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reported, where it usually adopts a “cone” conformation with all the phenolic oxygen atoms in a plane.7 The cavity is larger in diameter but shallower than that of p-sulfonatocalix[4]arene.8 In forming host guest and inclusion complexes, psulfonatocalix[5]arene can form bilayers with a distorted cone conformation.9 Interestingly the packing of 5-fold symmetry molecules into extended sheets while retaining this symmetry crystallographically is not possible. The iso-electronic pphosphonatocalix[5]arene is always isolated as an amorphous solid, forming lower rim to upper rim stacked fibers from solutions containing molecular capsules based on two calixarenes held together by a seam of hydrogen bonds.10 p-Sulfonatocalix[5]arene is similar to p-sulfonatocalix[4]arene in forming bilayers in the extended structures.11 A unique densely packed bilayer of p-sulfonatocalix[5]arenes forms in the presence of ytterbium(III) and tetraphenylphosphonium ions, with the latter acting as an auxiliary molecule in facilitating the crystallization of the metal complex of the calixarene.12 The calixarene can form complicated motifs such as the Ferris wheel-like coordination polymers involving complexation of the macrocycle with aza-crown ether and lanthanides.13 The presence of lanthanides is also important in the formation of a molecular capsule based on two p-sulfonatocalix[5]arenes in the cone conformation. Atwood et al. established the selfassembly in the solid state of several aquated lanthanide ions (Eu, Tb, Gd, and Yb) that bind to sulfonato oxygen atoms,14 while Su et al. have established complexation of bis-pyridinium in a molecular capsule comprised of two p-sulfonatocalix[5]-

INTRODUCTION The development of bowl-shaped molecules for recognition events is becoming more intriguing and of greater complexity, particularly for applications in biology.1 In general such molecules can accommodate molecular guests through complementary noncovalent interactions and can self-assemble into micelles or bilayers at the nanoscale.2 Calix[n]arenes are one of the most studied synthetic molecules for molecular recognition, where the size of the cavity can be varied, depending on the number of phenolic rings bridged by the methylene groups in the macrocycle. They have two segments, the so-called upper rim and lower rim, which can be modified chemically to provide higher structural and functional diversity. Calix[n]arenes bearing upper rim water solubilizing sulfonated groups have been extensively studied as supramolecular tectons in building complex multicomponent materials.3 Most of the studies focus on p-sulfonatocalix[4]arene, the smallest macrocycle in the family. A diverse range of guest molecules of different sizes and shapes, as neutral and charged species, can bind in the cavity of calixarenes, both in solution, and in the extended solid state structures.3,4 Apart from selective inclusion properties, the aforementioned molecule can self-organize into the simplest bilayer arrangement or form complex materials, which includes molecular capsules, “Russian dolls”, two- or three-dimensional (2-D or 3-D) coordination polymers, “Ferris wheels” and nanometer-scale dodecahedral arrays, in the form of icosahedra and cuboctahedra.3,5 There have been limited studies on the larger ring size p-sulfonatocalix[6 and 8]arenes which are conformational more flexible than the almost exclusively cone conformation for the calix[4]arene.6 For the synthetically difficult to access p-sulfonatocalix[5]arene, a few host−guest and inclusion complexes have been © 2012 American Chemical Society

Received: December 6, 2011 Revised: January 9, 2012 Published: January 11, 2012 1564

dx.doi.org/10.1021/cg201615u | Cryst. Growth Des. 2012, 12, 1564−1570

Crystal Growth & Design

Article

Figure 1. Schematic of the synthesis and associated encapsulation of bis-phosphonium endo-cavity relative to p-sulfonatocalix[5]arene.

arenes.15 p-Sulfonic acid calix[5]arene itself forms a hydrogen bonded molecular capsule, confining two sulfuric acid molecules.16 In the present study, we report a detailed analysis of the selfassembly of p-sulfonatocalix[5]arene complexed with large organic cations, namely, bis-phosphonium cations, in the presence of lanthanide(III) ions. Previously we carried out extensive studies on the complexation of p-sulfonatocalix[4]arene with imidazolium based cations of varying size and functionality, along with phosphonium cations, establishing that the imidazolium cations are selectively drawn into the cavity of the calixarene.17 We now find that the same approach involving p-sulfonatocalix[5]arene, for several imidazolium cations and selected organic phosphonium cations, results in crystalline material devoid of imidazolium cations, with bis-phosphonium cations taking on a dual role, both endo- or exo-relative to the cavity of the bowl shaped calix[5]arenes. The endo-interplay of the two components involves phenyl rings of the bisphosphonium cations with the calixarenes essentially arranged into “molecular capsules”, whereas for exo-interplay the cations are positioned within bilayers of calixarenes. 1H NMR solution studies to establish the nature of the interplay of the two different organic ions, and also imidazolium cations, at least in the absence of paramagnetic lanthanide ions were inconclusive, in contrast to analogous studies involving the smaller psulfonatocalix[4]arene where host−guest formation is evident for the imidazolium cations.18 This presumably arises from the lower complementarity of interaction of the imidazolium

cations with the larger shallower cavity of the more conformationally flexible p-sulfonatocalix[5]arene.

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RESULTS AND DISCUSSION A mixture of p-sulfonatocalix[5]arene, the large bis-phosphonium cation and several different imidazolium cations, either 1hexyl-3-methylimidazolium or 1-octyl-3-methylimidazolium, and gadolinium(III) chloride, afforded a complex based on three of the components, which vary only slightly in their water content, as established with the isolation of 1 and 2, respectively, Figure 1. These complexes have slightly different cell dimensions in the solid state, with subtle differences evident from their Hirshfeld surface analyses. In both structures two bis-phosphonium molecules are encapsulated by two calix[5]arenes, with the larger cations, that is, bis-phosphonium cations, drawn into the larger cavity of the calixarene, rather than the smaller imidazolium cations. Other bis-phosphonium cations are within a bilayer arrangement of the calixarenes. This finding contrasts with our previous finding for p-sulfonatocalix[4]arene where the charged headgroup of the imidazolium is preferentially drawn into the cavity in the solid state, and also in solution, yet similar with bis-phosphonium cations embedded in a bilayer arrangement of calixarenes.18 In the present study, the absence of the imidazolium cations results in the same complex. Complexes 1 and 2 crystallize in the triclinic space group with the asymmetric unit consisting of one calixarene anion (with disordered SO3− groups), two bis-phosphonium cations (with disordered phenyl rings), and one homoleptic 1565



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gadolinium(III) ion coordinated by eight water molecules, along with some disordered water molecules. However, the site occupancies of the [Gd(H2O)8]3+ is 1/3 for both complexes, noting that population of sites ≤1 is a common occurrence for inorganic cations included with p-sulfonatocalix[4]arene for lanthanide(III) and transition metal(III) ions (M3+ = Gd3+, Cr3+, Rh3+, Y3+, Eu3+, Pr3+, Nd3+).5a,c,19 In the extended structure, the sulfonated calixarenes are assembled in bilayers generated by skewed “molecular capsules” where the upper rims are aligned in a head-to-head fashion essentially providing a hydrophilic regions interposed with disordered water molecules, and both components are involved in mutual hydrogen bonding, as evidenced by short O···O contacts at 2.69(1)−2.98(2) Å involving [Gd(H2O)8]3+. The calixarenes take on an unusual conformation possessing a pseudo cavity (see below) with the bis-phosphonium cations located contiguously in two regions: (i) confined directly between the hydrophobic regions of the directionally opposed calixarenes (endo-cavity), and (ii) embedded in hydrophobic regions of the bilayers of calixarenes (exo-cavity), Figure 2. The

Figure 3. Coordination of gadolinium(III) in complex 2 showing a idistorted square antiprism arrangement.

capsules” are arranged in an intertwined network with other bis-phosphonium cations exo-relative to the calixarene cavity, in an expanded bilayer approximately 12.0 Å thick. A bilayer built exclusively of p-sulfonatocalix[5]arene in its sodium salt is 8.4 Å thick.12,14 There are small differences between complexes 1 and 2, mainly in relation to the interplay between the calixarenes and the bis-phosphonium cations. For the space between the central parts of opposing calixarenes in the “molecular capsules”, two phenyl rings of two centro-symmetrically related bis-phosphonium cations are positioned in the distorted-cavity of the same calixarene. One phenyl ring has close π···π interplay ring-centroid to ringcentroid distances at 4.24 and 4.00 Å respectively for complexes 1 and 2. The exo-cavity phosphonium cation has a phenyl ring with close C−H···π(centroid) contact to a calixarene methylene carbon atom, at 2.87 Å for both complexes, Figure 4

Figure 2. Structure of complex 1 showing the bilayer arrangement of calixarenes with embedded bis-phosphonium cations (blue) in between bis-phosphonium cations (green), viewed down the a-axis, with some space filling representation.

aquated metal ions have secondary coordination sphere interactions with the calixarenes associated with hydrogen bonding to sulfonate groups, evidenced by short GdO···OS distances at 2.86(2) and 2.90(3) Å, for complexes 1 and 2, respectively. The coordination around the Gd atom is the same for both complexes and is best described as a distorted square antiprism, Figure 3 with the opposite planes consisting of (i) O101, O102, O103, O108 and O104, O105, O106, O107 for complex 1 and (ii) O101, O102, O104, O107 and O103, O105, O106, O108 for complex 2. Similar coordination environments have been reported for lanthanide (M3+ = Gd3+, Tb3+, and Tm3+) complexes with p-sulfonatocalix[4]arene, with the metal centers a slightly distorted square antiprism arrangement of eight water molecules.20 The structures can be considered as being built of skewed “molecular capsules” based on two calixarenes, approximately 17 Å in height as measured along the principal axis, shrouding the ends of two bis-phosphonium cations. The “molecular

Figure 4. Combined stick-space filling projection of the bisphosphonium cations directed into the cavity of the calixarenes, showing the cation face-to-face π(centroid)···π(centroid) interaction (pink and purple colored phenyl rings with dotted lines).

(corresponding C···π(centroid) distance at 3.67 Å). The triphenylphosphine moieties and associated methylene bridges of the endo-cavity bis-phosphonium cations have weak hydrogen bonding to calixarene sulfonate groups, as judged by close C−H···O contacts at 2.48(1) to 2.85(1) Å (corresponding C···O distance at 3.16(1)−3.29(1) Å) and 2.36(1)−2.71(1) Å (corresponding C···O distance at 3.20(1)− 3.51(1) Å), for both complexes. Phenyl rings of two independent endo-cavity bis-phosphonium cations are in the phenyl embrace face-to-face arrangement, with π(centroid)··· π(centroid) distance at 4.06 Å for both complexes. 1566



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Figure 5. Distortion of the cone conformation for p-sulfonatocalix[5]arene associated with both endo- and exo-pseudo cavity interplay with the bisphosphonium cations.

The conformation of the p-sulfonatocalix[5]arene is perturbed from the usual cone conformation, with the aryl groups flattened, Figure 5; the dihedral angles between the phenyl plane and the basal plane containing the five methylene carbon atoms of the calixarene are summarized in Table 1.

Å (corresponding C···π(centroid) distances at 3.97 and 3.98 Å) for complexes 1 and 2, respectively. These cations are also involved in (i) hydrogen bonding with the O-atoms of the phenolic groups of the calixarene, with close C−H···O contacts at 2.531(4)−3.002(4) Å (corresponding C···O distances 3.345(8)−3.521(9) Å), (ii) C−H···π(centroid) interactions with the methylene moieties of the calixarenes, with the closest contacts ranging from 2.63 Å to 2.64 Å (corresponding CH···π(centroid) distances at 3.42−3.87 Å), and (iii) hydrogen bonding interactions with O-atoms of the calixarene sulfonate groups, with close contacts at 2.530(4)−2.996(4) Å (corresponding C···O distances 3.454(8)−3.515(9) Å). Hirshfeld Surface Analysis. In determining the nature of interplay of the different ionic species in the solid state, analysis of close intermolecular interactions has been undertaken using Hirshfeld surface analysis generated by Crystal Explorer.22 The parameter dnorm displays a surface with a red-white-blue color scheme where bright red spots are designated for contacts shorter than the sum of their van der Waals radii, while blue regions are devoid of close contacts.22 The analysis also provides an equivalent 2D-fingerprint plot that demonstrates the summary of the frequency of each combination of de and di across the surface of a molecule. Overall, the analysis shows subtle differences between the two complexes. Figure 7 compares the Hirshfeld surfaces and the corresponding fingerprint plots for calixarene and bis-phosphonium molecules for complexes 1 and 2. For the calixarene, the dominant intermolecular interaction is the C−H···π contacts, evident from the herringbone arrangement in the fingerprint plot. The nature of the C−H···π interactions between the calixarene and bis-phosphonium cation for complexes 1 and 2 are similar to each other, making up the large percentage contribution of the Hirshfeld surface (∼20% of overall interaction) with such interactions reflected as pairs of wings at the upper left and lower right of the fingerprint plot for the calixarene (areas circled in yellow), associated with the C−H···π donor and acceptor, respectively. The Hirshfeld surface mapped with dnorm reveals a bright red spot on the surface (yellow arrow) which is assigned to the close C−H···π contact involving the calixarene methylene bridge to the bis-phosphonium phenyl ring. For the bis-phosphonium component, short C−H···π contacts are also evident, which is reflected in the pair of spikes at the bottom left of the fingerprint plot (circled in yellow) for complexes 1 and 2 with the upper spike corresponding to the C−H···π donor, and the lower spike to the acceptor. Also, fingerprint plots for the bis-phosphonium component show symmetry along the diagonal of the plot which indicates a compact

Table 1. Dihedral Angles between the Aryl Plane and the Basal Plane of the Calixarene phenolic ring Ring Ring Ring Ring Ring

1 2 3 4 5

dihedral angle (o)

dihedral angle (o)

Complex 1: 82.2 31.6 29.2 29.3 47.7

Complex 2: 99.4 28.2 32.6 24.6 53.6

Departure from the cone conformation of p-sulfonatocalix[5]arene has been noted for inclusion complexes of 1,2bis(pyridinium)ethane and 1,4-diazabicyclo[2.2.2]octane, respectively, either as a partial-cone or where one of the sulfonate groups is directed in the opposite direction, as a pseudodouble cone conformation.9,15 The exo-cavity bis-phosphonium are involved in selfassociation through attractive edge-to-face (ef) interplay,21 Figure 6, with C−H···π(centroid) distances at 3.24 Å and 3.25

Figure 6. Projections highlighting the arrangement of the exo-cavity bis-phosphonium cations (green) relative to each other and the calixarene in forming the bilayer arrangement (for complex 1, with the same arrangement in 2). 1567



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Figure 7. Hirshfeld surface and equivalent fingerprint plots for calixarene and bis-phosphonium ions for complexes 1 (a and b), and 2 (c and d).

Figure 8. Relative contributions to the Hirshfeld surface for the major intermolecular contacts associated with the calixarene and bis-phosphonium ions in complexes 1 and 2.

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CONCLUSIONS Water-soluble p-sulfonatocalix[5]arene and a large bis-phosphonium cation in the presence of aquated gadolinium(III) ions form a multicomponent self-assembled material, at the exclusion of imidazolium cations present in solution. The material has a skewed molecular capsule arrangement of calixarenes shrouding the bis-phosphonium cations, and these are embedded in a bilayer arrangement with other bisphosphonium cations. Complex formation is associated with a perturbation of the calixarene from the usual more symmetrical cone conformation, and this conformational flexibility, and the preferential binding of larger cations, establish a design strategy for building complex materials based on the p-sulfonated calix[5]arene, for which its use in self-assembly processes has been limited. Moreover, the formation of the multicomponent system suggests that the larger p-sulfonatocalix[6 and 8]arenes are likely to form analogous structures with the calixarene able to vary its conformation to optimize the interplay with

arrangement of the ion with small intermolecular voids, and this is consistent with the evidence from the Hirshfeld surface where less blue area is observed. Hydrogen bonding involving hydrogen atoms and oxygen atoms for the calixarene and bis-phosphonium molecules are the major and important intermolecular interaction in both complexes. The H···H regions for both components (circled in red) show close H···H contacts for both complexes, with de + di around 2.1 Å for the calixarene and 2.0 Å for the bisphosphonium cation. The Hirshfeld surface of the calixarene and bis-phosphonium cations for both complexes reveals strong O···H hydrogen bonds highlighted by the red spots on the surface, and these contribute significantly to attractive interactions between crystal components with appreciable percentage contribution. Other close interactions for both components in complexes 1 and 2 are summarized as contribution percentages to the Hirshfeld surface area in Figure 8. 1568



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restrained to ideal values. One large peak in the difference map, approximately 1.1 Å from Gd1, was modeled as an oxygen atom of a water molecule disordered with the Gd(H2O)8 unit and assigned a site occupancy of 2/3. Remaining solvent atoms were modeled as water molecules with occupancies constrained to either 1.0 or 0.5 after trial refinement. Electron density that could not be reasonably modeled as such was effectively removed by use of the program Squeeze.27 Water molecule and hydroxyl group hydrogen atoms were not located. All remaining hydrogen 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. Atoms of the oxygen atoms of the minor components of the disordered sulfonate groups were refined with isotropic displacement parameters. Anisotropic displacement parameters were employed for the remaining nonhydrogen atoms.

phosphonium cations between the bilayers, and embedded in the bilayers, and similarly for large organic cations in general, including drug molecules.

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EXPERIMENTAL SECTION

p-Sulfonatocalix[5]arene, 1-hexyl-3-methylimidazolium, and 1-octyl-3methylimidazolium halide salts were synthesized according to the literature procedures.23 1,4-Bis(triphenylphosphoniomethyl)benzene chloride and gadolinium(III) chloride in the form of GdCl3·(H2O)6 were purchased from Sigma Aldrich and used as received. Crystals of 1 and 2 were grown by slow evaporation of an acidified solution (using 1 M HCl) containing equimolar amounts (0.01 M) of calixarene, imidazolium salts (1-hexyl-3-methylimidazolium chloride and 1-octyl3-methylimidazolium chloride respectively), bis-phosphonium salt and gadolinium(III) chloride. On standing over several days, small colorless crystals suitable for X-ray diffraction studies deposited. Crystallography. All data were measured using an Oxford Diffraction Gemini CCD diffractometer at T = 100(2) K with monochromatic Mo−Kα radiation (λ = 0.71073 Å) for 1 and Cu−Kα radiation (λ = 1.54178 Å) for 2. The data were corrected for Lorentz and polarization effects, with absorption corrections applied using multiple symmetry equivalent reflections. The structures were solved by direct methods and refined using full matrix least-squares on F2 with the SHELX-9724 crystallographic package, via the WinGX,25 and X-seed26 interfaces. Crystal/Refinement Details for Complex 1. 2(C44H38P22+), C35H25O20S55−, 0.33(H16GdO83+), ∼11.85(H2O), C123H130Gd0.33O34.50P4S5, M = 2496.87, colorless prism, 0.71 × 0.23 × 0.10 mm3, triclinic, space group P1̅, a = 17.3959(6), b = 18.1202(5), c = 25.2906(7) Å, α = 72.424(3)°, β = 70.744(3)°, γ = 63.200(3)°, V = 6605.6(3) Å3, Z = 2, Dc = 1.255 g/cm3, μ = 0.374 mm−1, 2θmax = 60.94°, 69 892 reflections collected, 35 804 unique (Rint = 0.0478). Final GOF = 1.018, |Δρmax| = 2.47 e Å−3, R1 = 0.1021, wR2 = 0.2962, R indices based on 16 424 reflections with I > 2σ(I), 1693 parameters, 365 restraints. CCDC deposition number: 855366. The sulfonate oxygen atoms on groups 1, 4, and 5 were modeled as being disordered over two sets of sites with occupancies constrained to 0.5 after trial refinement. Geometries were restrained to ideal values. The site occupancies of the atoms of the [Gd(H2O)8]3+ unit were constrained to 1/3 on the basis of charge balance requirements and also from trial refinement. One phenyl ring of one of the bisphosphonium cations was also modeled as disordered over two sites with occupancies constrained to 2/3:1/3 after trial refinement. Residual electron density was modeled as oxygen atoms of water molecules with site occupancies constrained to unity or, for those with close approaches to disordered groups, 2/3 or 0.5. These occupancies were consistent with values obtained by trial refinement. The remaining electron density could not be modeled sensibly as water molecules, even with partial occupancies, and was therefore effectively removed by use of the program Squeeze.27 The displacement parameters of the disordered groups were restrained to realistic values. Water molecule hydrogen atoms were not located. Crystal/Refinement Details for Complex 2. 2(C44H38P22+), C35H25O20S55−, 0.33(H16GdO83+), ∼11.70(H2O), C123H129.67Gd0.33O34.33P4S5, M = 2493.86, colorless block, 0.24 × 0.16 × 0.11 mm3, triclinic, space group P1̅, a = 17.4153(9), b = 18.1333(13), c = 25.2944(14) Å, α = 72.365(5)°, β = 70.778(5)°, γ = 63.287(6)°, V = 6622.9(7) Å3, Z = 2, Dc = 1.251 g/cm3, μ = 2.928 mm−1, 2θmax = 134.3°, 65 590 reflections collected, 23 329 unique (Rint = 0.0736), final GOF = 1.045, |Δρmax| = 1.98 e Å−3, R1 = 0.1005, wR2 = 0.2806, R indices based on 13942 reflections with I > 2σ(I), 1689 parameters, 159 restraints. CCDC deposition number: 855367. The site occupancies of the atoms of the [Gd(H2O)8]3+ cation were constrained to 1/3 on the basis of trial refinement and charge balance considerations. Two phenyl rings of one bis-phosphonium cation and the oxygen atoms of two sulfonate groups on the calixarene anion were modeled as being disordered with site occupancies of each component constrained to 2/3 and 1/3 after trial refinement. Geometries of the atoms of the minor components of the disordered atoms were

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ASSOCIATED CONTENT

S Supporting Information *

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

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AUTHOR INFORMATION

Corresponding Author

*Fax: +603 79674193, +61 864881005. E-mail: ireneling@ siswa.um.edu.my (I.L.) [email protected] (C.L.R.).

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ACKNOWLEDGMENTS We thank the University of Malaya (HIR-MOHE F00000421001 and RG062-909SUS), The University of Western Australia, and the Australian Research Council for supporting this work. The authors acknowledge the facilities, 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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