arene Bilayer Relative to the Ubiquitous Sulfonato-calix[4] - American

Atwood et al. have reported the formation of several p-sulfonato-calix[5]- arene/lanthanide/pyridine-N-oxide complexes, which typically crystallize as...
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Efficiency of Packing the Sulfonato-calix[5]arene Bilayer Relative to the Ubiquitous Sulfonato-calix[4]arene Analogue Mohamed Makha* and Alexandre N. Sobolev School of Biomedical, Biomolecular and Chemical Sciences, UniVersity of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 8 1441-1445

ReceiVed NoVember 27, 2006; ReVised Manuscript ReceiVed March 27, 2007

ABSTRACT: Ytterbium(III) forms a complex with sulfonato-calix[5]arene in water with the calixarene assembled into the common clay bilayered arrangement but having unusually dense packing of the calixarenes, which are squeezed together forming “impenetrable” bilayers. The efficiency of packing of the calixarene relates to its conformational flexibility, and it has implications in providing a protective layer in stabilizing spheroidal nanoparticles. Introduction There is an ever-growing interest in the supramolecular assembly of sulfonato-calix[n]arenes (n ) 4, 5, 6, 8) displaying a plethora of solid-state architectures and imparting information of significance in materials and of biological relevance.1 The amphiphilic nature of these water-soluble calixarenes is manifested in a wide range of metal ion binding and remarkable inclusion properties.2 They are widely used in supramolecular chemistry as building blocks for crystal engineering,1,2 in catalysis,3 and in the stabilization of nanoparticles.4 The most popular member of this family of compounds is sulfonato-calix[4]arene, which relates to its rigid structure and predisposition to take the cone conformation. While higher ring sizes such as sulfonato-calix[6,8]arenes display flexibility with greater degree of conformational freedom,5 sulfonato-calix[5]arene with its truncated cone structure and larger cavity present an alternative to its smaller sibling. The supramolecular chemistry of sulfonato-calix[5]arene is not well explored with only six structures to date in the Cambridge Database.6-11 The reason for this scarcity relates not only to its availability but also to the challenges in crystallizing its complexes. The latter presumably is due to the pseudo 5-fold local symmetry and associated difficulties in packing molecules of this symmetry in continuous arrays and its extra degree of conformational freedom. This can have a profound effect on the packing of the calixarene into the bilayer arrangement. Indeed, crystallization of the calixarene generally requires auxiliary components (small organic molecules and metal cations) to generate the bilayer assembly process in the solid state. Nevertheless, the sulfonic acid of the calix[5]arene can be crystallized from concentrated sulfuric acid solution forming a dimeric capsule with a sulfuric acid molecule residing in each calixarene cavity.6 In this case, the pseudo bilayer formation is possibly assisted by the intercalation of sulfuric acid molecules via hydrogen bonding to the sulfonic acid groups and effectively networking the hydrophilic domain. Atwood et al. have reported the formation of several p-sulfonato-calix[5]arene/lanthanide/pyridine-N-oxide complexes, which typically crystallize as “molecular capsule” type arrangements.7 Also reported are lanthanide metal complexes that relate to the present study where sulfonato-calix[5]arene units adopt a bilayer with the La(III) ions, along with sodium ions, and water residing in the hydrophilic layer.7 Although sulfonato-calix[5]arene has limitations in its synthetic procedure, it is an attractive host molecule given the larger

size of its cavity relative to sulfonato-calix[4]arene. Raston et al. recently reported the complexation of sulfonato-calix[5]arene with small organic molecules, including DABCO and crown ethers, in the presence of lanthanide(III) ions.8-10 Reports also include the formation of lanthanides/(di)aza-crown ether/sulfonato-calix[5]arene complexes that consist of either a hostguest arrangement or a Ferris wheel-like coordination polymer.10 While sulfonato-calix[4]arene with the 4-fold symmetry is known to form bilayers by hydrophobic association of the calixarenes in an “up/down” arrangement, sulfonato-calix[5]arene with the 5-fold symmetry appeared less likely to pack closely in a bilayer arrangement in the symmetrical rigid cone conformation, Figure 1. Herein we report the formation of a sulfonato-calix[5]arene/ ytterbium/sodium complex, which consists of a compact and discrete bilayer arrangement formed through bridging of the sulfonate groups by ytterbium and sodium cations with increased bilayer thickness. The complex forms only in the presence of an organic molecule and without these auxiliary molecules taking part in the final product. It is intriguing that in the absence of the organic molecules no crystalline material forms, suggesting that these auxiliary molecules serve as inducers for the crystallization process. The overall structure features a discrete coordination bilayer, in addition to a dense packing of bilayers. Similar packing of the sulfonato-calix[5]arene is common in all the reported structures to date for this calixarene.7,11 For comparison purposes, four lanthanide analogous structures of sulfonato-calix[4]arene were prepared and characterized featuring the usual bilayers bridged by sodium cations. The intricate interplay of the sulfonato-calix[5]arene metal complex effectively creates impenetrable bilayers. We have recently demonstrated the utility of this efficient mode of packing for the calix[5]arene in the stabilization of carotenoid nanopaticles.4 The tight packing of this calixarene in the bilayer blocks osmium tetroxide in breaching the calixarene surface coating and dissolution of the carotenoid in organic solvents. Results and Discussion Although sulfonato-calix[5]arene seemingly has limitations in its complex formation and supramolecular chemistry, we have obtained reasonably good crystals suitable for X-ray structure analysis of the yetterbium complex: sulfonato-calix[5]arene‚ Yb‚Na2‚(H2O)14. The complex crystallizes in the monoclinic space group P21/c, Z ) 8, with the asymmetric unit comprised of two crystallographically independent calixarenes from two

10.1021/cg060848k CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007

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Figure 1. Top and side view projections of sulfonato-calix[4 and 5]arenes showing the cone conformation and a cartoon describing the modes of packing of the calixarenes through π-stacking when regarded as rigid cones: red and blue polygons represent calixarenes in opposite directions.

Figure 3. Projection of the bilayer structure of sulfonato-calix[5]arene/ Yb/Na2‚(H2O)14 viewed along the crystallographic z-axis (top) and space fill for a discrete bilayer within the structure (bottom) (lattice water is omitted for clarity).

Figure 2. Asymmetric unit showing the two calixarenes and sulfonate bound aquated ytterbium (purple) and sodium cations (brown) (water of crystallization is omitted for clarity and coordination spheres of Yb and Na atoms completed with symmetry-related oxygen atoms).

separate bilayers. The ytterbium center shares the sulfonate group with an aquated sodium center in trans configuration. Each sodium cation is bound to a sulfonate group and additional second coordination sphere to water solvent molecules. The calixarene molecules within the bilayer form an infinite corrugated polymeric structure, where each calixarene is bound through the oxygen atoms of sulfonato groups to a penta- or hexa-aquated ytterbium(III) cation to complete the metal center coordination sphere (Yb‚‚‚O 2.26(2)-2.38(2) Å). Four sodium cations have different oxygen coordination environment that can be expressed as NaO4(OH2)2, NaO3(OH2)3, NaO3(OH2)2, and [(OH2)O4Na-NaO4(OH2)], (Na·‚‚O 2.29(2)-2.88(2) Å). The other calixarene in the asymmetric unit has the ytterbium unicoordinated to a sulfonate group with one water molecule in the second coordination sphere, while the two sodium centers have similar unicoordination to adjacent sulfonato groups

(Figure 2). One of the calixarene has two crystalline water molecules deeply embedded in the cavity with a short O‚‚‚O distance of 2.75 Å. The cavity of the other calixarene is devoid of water molecules with only one water molecule at the hydrophilic region associated with the second coordination sphere of the ytterbium cation. Intricate hydrogen bonding is formed between water molecules and part of the aquated sphere of the ytterbium center which leans toward the cavity and is hydrogen bonded to one oxygen of the opposite sulfonato group. The geometries of the hydrogen bonds are recorded in Supporting Information. Despite the distortion of cone conformation the calixarene retains the H-bonded network at the lower rim, with short O‚ ‚‚O contacts varying from 2.70 to 3.70 Å (O·‚‚O distances 2.74, 3.73, 2.71, 3.84, 2.75 Å in calixA; and 3.53, 2.75, 2.86, 3.31, 2.91 Å in calixB). Both calixarene molecules adopt a distorted pinched cone conformation with the angle of the phenyl rings relative to the plane of the five oxygen atoms varying from 88.6° to 166.2° (calix A) and from 93.9° to 155.30° (calix B) (Figure 2). These values compare well with other structures of sulfonato-calix[5]arenes with the exception of two instances where the calixarenes take on unusual conformations.9,12 The sulfonato-calix[5]arene units form a bilayer structure, as depicted in Figure 3, which is commonly observed for a number

Packing Sulfonato-calix[5]arene Bilayer

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Figure 4. Top view projection and space fill of the bilayer of sulfonato-calix[4]arene showing the usual up and down arrangement of the calixarenes and the channels formed by inefficient packing; (left) the analogous bilayer of sulfonato-calix[5]arene showing dense packing arrangement (right) (highlighted calixarenes with different colors in both bilayers showing the up and down arrangement; water and metal cations are omitted for clarity).

solid-state structures of sulfonato-calix[4,5]arenes.6-11 The hydrophilic regions consist of intercalation of metals cation bound to the sulfonate groups, which results in expanding the thickness of the bilayer ca. 9.4 Å compared to 8.4 Å for the sodium sulfonato-calix[5]arene.11,13 It is noteworthy that similar expansion with the sulfonato-calix[4]arne is associated with intercalation of organic cations (Ph4P+).14 Within the bilayer aromatic CH‚‚‚π, π‚‚‚π interactions and hydrogen bonding between a sulfonate group of one calixarene to the hydroxyl of another adjacent calixarene with a SO‚‚‚O short contact of 2.60 Å (Figure 3). To substantiate this difference in the packing arrangement of the bilayers in sulfonato-calix[4,5]arenes, we have structurally authenticated four sodium sulfonato-calix[4]arene lanthanide complexes with Yb, Er, Gd, and Tm(III) and closely looked at their structural composition and organization (Figure 4). All complexes are comprised of centrosymmetric dimers where two sodium cations bridge the nearest neighboring sulfonate groups of two calixarenes. The four compounds are isostructural and crystallize in the triclinic space group P1h, Z ) 1. The refinement model of the Gd complex was used as a template for other three structures, and the Gd complex is a complete analogue of the Tm complex. Similarly, the Er(III) and Yb(III) complexes are identical but differ from the Gd(III) and Tm(III) complexes based on the number of water molecules in the unit cells. In the Yb(III) complex, three water molecules of crystallization in the asymmetric unit have site occupation of 0.5, and one of the sulfonate groups of the calixarene is disordered between two position with a ratio 0.5:0.5. Moreover, the sulfonate group in the same position for the Er(III) complex is disordered according to the shape of the atomic displacement parameters ellipsoids of the oxygen atoms. The two pairs of sulfonato-calix[4]arene lanthanide complexes in this series differ in the number of water molecules of crystallization in the structures. The major structural differences in these pairs include the orientation of water molecule residing almost at the center of the calixarene cavity and the nature of the “disorder” of one of the sulfonate groups. In the Er/Yb structures, the deficiency in crystalline water molecules is directly related to the presence or the absence of water molecules in certain asymmetric units within the molecular crystal. Namely, one “disordered” part of the sulfonate group in the Er/Yb systems has oxygen atoms involved in hydrogen bonding with water molecules of fully occupied asymmetric units. The second part of the “disorder” is twisted with respect to part 1

(about 30° in the Yb(III) complex relative to S-C bond) and do not show hydrogen bond distances to the ghostly crystalline water molecules. Thus, the disorder of the sulfonate groups and partial occupancy of some crystalline water molecules are related events and is seemingly the cause for discrepancies in the asymmetric unit molecular contents. Despite the experimental X-ray data hampering locating the hydrogen atoms positions for water molecules with confidence from difference Fourier maps, we reconstructed their positions from analysis of the shortest O‚‚‚O contacts, which were attributed to O-H‚‚‚O hydrogen bonding interactions. The detailed analysis of all possible hydrogen positions of water molecules showed the existence of only a unique combination where the hydrogen atoms do not overlap with other closest hydrogen atoms from other molecules, and all hydrogen atoms of water molecules are involved in a H-bond network. This model supposes that one crystalline water molecule located almost at the middle of the calixarene cavity will have two hydrogen atoms pointed to the centeroids of two opposite aromatic rings forming O-H‚‚‚π interactions, as reported by Atwood.1a This model was used for all the four lanthanide complexes of sulfonato-calix[4]arene presented in this work. The shortest O-H‚‚‚π contacts are 3.41 and 3.55 (2), 3.35 and 3.60 (3), 3.41 and 3.54 (4), and 3.37 and 3.62 A (5). Details of the O-H‚‚‚O hydrogen bond network are in Supporting Information. We have recently reported the surface coating of carotenoid nanoparticles by sulfonato-calix[n]arenas (n ) 4-8) as a process for their stabilization.4 Transmission electron microscopy (TEM) analysis showed clear structural differences in these nanoparticles produced in the presence of sulfonato-calix[n]arenes. The nanoparticle samples were treated with osmium tetroxide (OsO4) to provide enhanced contrast for TEM imaging via reaction of OsO4 with the conjugated system of the carotenoid. All samples exhibited staining except only for the samples prepared using sulfonato-calix[5]arene where the staining of the particles was circumvented, and thus the compact “surfactant” layer protects the carotenoid particles. For comparison, we prepared a similar compound based on sulfonato-calix[4]arene and evaluate the bilayers packing efficiency. Close inspection of both bilayer surfaces revealed a restricted surface for sulfonato-calix[5]arene, and this then provides a possible explanation for the variations observed in the movement of OsO4 across the outer calixarene layer (Figure 5). The special case of sulfonato-calix[5]arene affording particles mostly stable toward OsO4 attack suggests

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Figure 5. Top and side view projection in ball and stick of packing diagram of the bilayer of sulfonato-calix[4]arene showing the usual up and down arrangement of the calixarenes and the channels formed by inefficient packing; (a) the bilayer of sulfonato-calix[5]arene showing the dense packing of the bilayer devoid of channels (ytterbium and sodium metal along with water molecules are omitted for clarity).

that packing of potentially 5-fold symmetry calixarenes provides a tightly assembled ordered protective outer layer. Packing of potentially 5-fold symmetry molecules is efficient in the case of forming icosahedra, but this is for monolayers of the calixarenes forming ca. 3 nm particles.15 For the larger nanoparticles ca >30 nm, the size of the calixarene molecules relative to the size of the particles implies that for the nanoparticles the packing of the calixarenes is essentially on a flat surface. We also note that carotenoid nanoparticles shrouded by sulfonated calix[5]arene are stable in the presence of organic solvents.4 Conclusion We have structurally authenticated a metal complex based on sulfonato-calix[5]arene and through critical analysis established the dense bilayer formation. Similar bilayers are abundant, and linking the structural organization to the surface property of these bilayers is a challenge. The densely packed bilayer arrangement of sulfonato-calix[5]arenes is noteworthy in the ability to replicate such organization on surfaces of nanoparticles. This has implications in building up material for application in many areas such drug stabilization, delivery systems, and surface science, a trajectory we are currently pursuing. Experimental Section Sulfonato-calix[4,5]arenes were prepared as described previously in the literature, and all starting materials and solvents were purchased commercially and used as supplied. The ytterbium metal complex of sulfonato-calix[5]arene was prepared by slow evaporation of aqueous solutions containing the sodium salt calix[5]arene sulfonate with an excess of ytterbium chloride in the presence of tetraphenylphosphonium cations, with crystals suitable for single-crystal diffraction studies forming within two weeks. The lanthanide metal complex of sulfonato-calix[4]arene were prepared by slow evaporation of aqueous solutions containing the sodium salts calix[4]arene sulfonate with an excess of appropriate lanthanide chloride (Yb, Gd, Tm, Er), with crystals suitable for singlecrystal diffraction studies forming within a week. Crystallography. The X-ray diffracted intensities were measured from a single crystals at about 100 K on an Oxford Diffraction Xcalibur-S and at about 153 K on a Bruker ASX CCD diffractometers

using monochromatized Mo-KR (λ ) 0.71073 Å.) Data were corrected for Lorentz and polarization effects and absorption correction applied using multiple symmetry equivalent reflections. The structures were solved by direct method and refined on F2 using SHELX-97 crystallographic package. A full matrix least-squares refinement procedure was used, minimizing w(Fo2 - Fc2), with w ) [σ2(Fo2) + (AP)2 + BP]-1, where P ) (Fo2 + 2Fc2)/3. Agreement factors (R ) ∑||Fo| - |Fc||/ ∑|Fo|, wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2 and GOF ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2 are cited, where n is the number of reflections and p is the total number of parameters refined). All non-hydrogen atoms of the non-disordered groups were refined anisotropically, while the disordered non-hydrogen atoms were refined isotropically. The positions of hydrogen atoms were calculated from geometrical consideration. The hydrogen atomic parameters were constrained to the bonded atoms during the refinement. CCDC 627582-627586 1. Crystal/Refinement Details for C35H25O20S55-, Na2+, Yb3+, 14(H2O). C35H53Na2O34S5Yb, M ) 1397.09, F(000) ) 5656 e, monoclinic, P21/c, Z ) 8, T ) 100(2) K, a ) 24.8125(5), b ) 18.8507(3), c ) 21.8656(3) Å, β ) 93.077(2)°, V ) 10212.5(3) Å3; Dc ) 1.817 g cm-3; sin θ/λmax ) 0.5969; N(unique) ) 18014 (merged from 157914, Rint ) 0.1745, Rsig ) 0.1119, No (I > 2σ(I)) ) 15253; R ) 0.1781, wR2 ) 0.3706 (A,B ) 0.015, 1500.0), GOF ) 1.235; |∆Fmax| ) 3.8(3) e Å-3, sample size 0.40 × 0.23 × 0.03 mm. 2. Crystal/Refinement Details for [(C28H20O16S4)4-, Na+(OH2)4, Gd3+(OH2)7]2, 14(H2O). C56H112Gd2Na2O68S8, M ) 2490.42, F(000) ) 1270 e, triclinic, P1h, Z ) 1, T ) 153(2) K, a ) 12.253(4), b ) 13.641(4), c ) 14.306(5) Å, R ) 90.274(7), β ) 105.504(7), γ ) 91.368(7) °, V ) 2303.3(13) Å3; Dc ) 1.795 g cm-3; sin θ/λmax ) 0.7035; N(unique) ) 12 579 (merged from 23274, Rint ) 0.0578, Rsig ) 0.0865), No (I > 2σ(I)) ) 9818; R ) 0.0844, wR2 ) 0.2473 (A,B ) 0.141, 29.7), GOF ) 1.089; |∆Fmax| ) 10.2(3) e Å-3, sample size 0.25 × 0.12 × 0.10 mm. 3. Crystal/Refinement Details for [(C28H20O16S4)4-, Na +(OH2)4, Er3+(OH2)7]2, 11(H2O). C56H106Er2Na2O65S8, M ) 2456.39, F(000) ) 1248 e, triclinic, P1h, Z ) 1, T ) 100(2) K, a ) 12.1676(6), b ) 13.4847(7), c ) 14.119(4)Å, R ) 90.164(9), β ) 105.282(11), γ ) 91.581(4)°, V ) 2233.7(7) Å3; Dc ) 1.826 mg m-3; µMo ) 2.181 mm-1, sin θ/λmax ) 0.7035; N(unique) ) 12 969 (merged from 33498, Rint ) 0.0399, Rσ ) 0.0808), No (I > 2σ(I)) ) 9522; R ) 0.0468, wR2 ) 0.1100 (A,B ) 0.059, 1.5), GOF ) 1.016; |∆Fmax| ) 2.1(1) e Å-3, sample size 0.20 × 0.10 × 0.07 mm. 4. Crystal/Refinement Details for [(C28H20O16S4)4-, Na+(OH2)4, Tm3+(OH2)7]2, 14(H2O). C56H112Na2O68S8Tm2, M ) 2513.78, F(000) ) 1280 e, triclinic, P1h, Z ) 1, T ) 153(2) K, a ) 12.179(2), b ) 13.615(3), c ) 14.371(3) Å, R ) 90.300(4), β ) 105.180(4), γ ) 91.490(4)°, V ) 2298.9(8) Å3; Dc ) 1.816 g cm-3; sin θ/λmax ) 0.7035;

Packing Sulfonato-calix[5]arene Bilayer N(unique) ) 13 046 (merged from 21498, Rint ) 0.0850, Rsig ) 0.0865), No (I > 2σ(I)) ) 11793; R ) 0.1009, wR2 ) 0.2652 (A,B ) 0.123, 64.7), GOF ) 1.143; |∆Fmax| ) 16.4(4) e Å-3, sample size 0.45 × 0.18 × 0.16 mm. 5. Crystal/Refinement Details for [(C28H20O16S4)4-, Na+(OH2)4, Yb3+(OH2)7]2, 11(H2O). C56H106Na2O65S8Yb2, M ) 2467.95, F(000) ) 1252 e, triclinic, P1h, Z ) 1, T ) 100(2) K, a ) 12.1812(13), b ) 13.5222(8), c ) 14.0649(16) Å, R ) 90.365(7), β ) 104.904(11), γ ) 91.504(7)°, V ) 2237.8(4) Å3; Dc ) 1.831 g cm-3; sin θ/λmax ) 0.7035; N(unique) ) 12995 (merged from 66548, Rint ) 0.0397, Rsig ) 0.0523), No (I > 2σ(I)) ) 9861; R ) 0.0682, wR2 ) 0.1891 (A,B ) 0.137, 2.085), GOF ) 1.042; |∆Fmax| ) 10.7(3) e Å-3, sample size 0.20 × 0.10 × 0.07 mm.

Acknowledgment. We thank the Australian Research Council and the University of Western Australia for support of the work. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

Crystal Growth & Design, Vol. 7, No. 8, 2007 1445

(3) (4) (5)

(6) (7)

(8) (9) (10)

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