Supramolecular Bracelets and Interlocking Rings Elaborated Through

Aug 5, 2008 - South Wales @ ADFA, Australian Defence Force Academy, Canberra, ACT 2600, Australia. ReceiVed May 2, 2008; ReVised Manuscript ...
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CRYSTAL GROWTH & DESIGN

Supramolecular Bracelets and Interlocking Rings Elaborated Through the Interrelationship of Neighboring Chemical Environments of Alkyl-Substitution on Cucurbit[5]uril

2008 VOL. 8, NO. 9 3446–3450

Xin-Long Ni,† Jing-Xiang Lin,† Yu-Ying Zheng,‡ Wen-Shi Wu,⊥ Yun-Qian Zhang,† Sai-Feng Xue,† Qian-Jiang Zhu,† Zhu Tao,*,† and Anthony I. Day*,# Key Laboratory of Macrocyclic & Supramolecular Chemistry of Guizhou ProVince, Guizhou UniVersity, Guiyang, Guizhou, 550025 China, College of Chemistry & Chemical Engineering, Fuzhou UniVesity, Fuzhou, Fujian, 350002 China, College of Material Science & Engineering, HuaQiao UniVersity, Quanzhou, Fujian, 362011, China, and Chemistry, School of PEMS, UniVersity of New South Wales @ ADFA, Australian Defence Force Academy, Canberra, ACT 2600, Australia ReceiVed May 2, 2008; ReVised Manuscript ReceiVed June 18, 2008

ABSTRACT: The smallest members of the cucurbituril family, cucurbit[5]uril and the alkyl-cucurbit[5]urils, can be used as a building blocks, linked by metal ions to create supramolecular rings. The cavities found at the center of these rings have dimensions between 7 and 19 Å in width and 8.5 Å in depth. The partially substituted alkyl-cucurbit[5]urils present the most interesting supramolecular ring formation. This occurs as a result of selective coordination of metal ions to the carbonyl oxygens of the glycoluril moieties carrying alkyl substitution. Introduction The basic unsubstituted cucurbituril family contains five members with cucurbit[5]uril, Q[5], being the smallest member or homologue.1 The first reported Q[5] was the permethylated Me10Q[5].2 This substituted Q[5] was structurally identified as a lone member of its type, 8 years before the discovery of the unsubsituted Q[n] family.1 Few examples of persubstituted Q[5]’s have been reported since the discovery of the unsubsituted Q[n] family, and the chemistry of the Q[5]’s has received little attention compared to the larger homologues.1 Primarily, the Q[5]’s appear limited chemically by the size of the portals and the capacity of their cavities.1 Q[5]’s have been shown to bind small molecules, such as CH4, C2H4, N2, O2, CO2, methanol and acetonitrile as some examples.3–10 In addition, anion encapsulation has been demonstrated2 and in the case of the Cl- ion, selective encapsulation over the NO3- ion with La3+ at the portal.11,12 Portal ion-dipole interactions are also important and selective for Q[5]’s.1,13–16 Of particular note is the high selectivity of Me10Q[5] for Pb2+ over alkali, alkalineearth, NH4+, and Cd2+ cations as determined by calorimetric and potentiometric methods. The selectivity factor in this case was >105.5.13 The unsubstituted Q[5] also serves as a molecular guest encapsulated in the cavity of Q[10].17 Much attention has been focused on the design and synthesis of interlocked molecules, such as rotaxanes, catenanes, and molecular knots.18–21 The attraction being aesthetics as well as the potential for nanoscale applications toward molecular devices and new materials.18 In Q[n] chemistry, these types of structures were first realized by molecular threading of Q[6] to form rotaxanes through coordination to transition metals and further to catenanes.1 This has since been extended to Q[8] and to a lesser extent Q[7]. The intrinsic ability of Q[8] to encapsulate two molecules at once has been utilized to form such structures through host-stabilized intermolecular CT complexes.22 * To whom correspondence should be addressed. E-mail: (A.I.D) a.day@ adfa.edu.au; (Z.T.) [email protected]. † Guizhou University. ‡ Fuzhou University. ⊥ HuaQiao University. # University of New South Wales @ ADFA.

Other significant developments in Q[n] chemistry have been the construction of discrete cyclic nanostructures by a modular approach based on coordination interactions of transition metal ions with the portal carbonyls of each Q. These metal ions have high affinity and selectivity in solution and in the solid state. Many X-ray crystal structures of Q[6] linked in this way by various metal ions have been reported.1,23,24 The potential for the formation of interlinked structures fashioned in a similar way but based on the smallest members of the Q[n] family was high. It was anticipated that the noncovalent interconnection between two adjacent portals and a metal ion would achieve a string of Q[5]’s, as had occurred with other Q[n]. As an added dimension, it was expected that the interplay between alkyl-substituents decorating the girth of a Q[5] and a neighboring Q[5] similarly decorated, together with water molecules or ions, would lead to novel solid state structures or MOF’s. These structures were expected to create molecular space with useful sizes (5-10 Å or larger). An added dimension was to use only partially alkylated Q[5]’s and not peralkylated. Experimental Section All the reagents and solvents were commercially available and used without further purification. Preparation of {K2(H2O@Q[5])}[InCl4 · 2H2O]Cl · 4.5H2O. A solution of InCl3 (1.0 g) in water (5 mL) was added to saturated aqueous solution of Q[5]KCl (1.2 g) and the mixture was heated in a water bath for 0.5 h. During the process of cooling to room temperature, 0.5 mL of 32% HCl was add to the solution. Finally the solution was filtered and allowed to slowly evaporate in air. Crystals formed over a period of 1 week. Preparation of {Sr2(Cl@r,r′-DMeQ[5]}3Cl · 19H2O. A solution of R,R′-DMeQ[5] (1 g, 0.6 mmol) and SrCl2 (1 g, 3 mmol) in water (15 mL) was refluxed for 24 h. The resulting solution was filtered and set aside to allow crystals to grow at room temperature. Colorless X-ray quality crystals (0.65 g) were obtained from the solution after several weeks. Preparation of {K2(H2O@r,β,δ-TriCyHQ[5])}2Cl · 15.5H2O. A solution of R,β,δ-TriCyHQ[5] (see Supporting Information for synthesis) (1 g, 0.6 mmol) and KCl (0.3 g, 3 mmol) in water (15 mL) was refluxed for 12 h. The resulting solution was cooled and set aside to allow crystals to grow at rt. Colorless X-ray quality crystals (0.16 g) were obtained from the solution over ∼4 weeks.

10.1021/cg800451z CCC: $40.75  2008 American Chemical Society Published on Web 08/05/2008

Alkyl-substituted Cucurbit[5]uril in Rings

Figure 1. Crystal structures of rings formed from Q[5], R,R′-DMeQ[5], and R,β,δ-TriCyHQ[5] (top view) space-filling representations and stick models for the X-ray crystal structures of (a) the 8-membered “beaded” ring, (b) the 6-membered “beaded” ring, (c) the 10-membered “beaded” ring. Hydrogen atoms, solvent water molecules are omitted for clarity. Color codes: carbon, gray; nitrogen, blue; oxygen, red; K ion, purple; cyclohexanyl rings orange; methyl groups yellow. The mean diameter of each ring formed is (a) ∼14.8 Å, (b) ∼7.7 Å, (c) ∼19.3 Å, respectively.

Crystallographic data for the structures reported have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-647405, 666616, 684922. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Results and Discussion Three types of novel Q[5] based solid state structures were found which resemble molecular bracelets “beaded” with 6, 8, and 10 Q[5] “beads”. The comparison between unsubstituted Q[5] and two partially decorated alkyl Q[5]’s, which were dimethyl- and tricyclohexano- abbreviated as R,R′-DMeQ[5] and R,β,δ-TriCyHQ[5], respectively, resulted in elaborate structures of Q’s linked in strings arranged in rings. The rings are favored by the placement of the alkyl groups relative to the anion and water molecules. Significantly, direct coordination of metal ions to the carbonyl oxygen of the substituted glycoluril moieties of two different Q is also relevant to ring formation (Figure 1b,c). The eight-membered “beaded” ring (Figure 1a) is formed from unsubstituted Q[5] with the portal coordination markedly different to that of the substituted Q[5]. Unlike the substituted Q[5] examples, coordination between two adjacent portals is direct portal carbonyl O to a K+ ion and then interlinked to the neighboring portal through the coordination sphere of shared water coordinated to a K+ ion directly coordinated to the portal of the next Q[5] to form strings of Q[5] “beads”. Interestingly, this was found to occur in the presence of the large InCl4counterion. Previously reported Q[5] metal ion coordinated

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Figure 2. The structure of (a) Q[5] molecular capsule. (b) 8-membered Q[5] “beaded” rings, top view. (c) Relative relationship of [InCl4 (H2O)2] to Q[5] (d) 8-membered Q[5] “beaded” rings, side view.

structures with Cl- as the counterion form linear assemblies and not rings of Q “beaded” strings.11,12 The crystals obtained from a HCl solution of Q[5], KCl, and InCl3 had a stoichiometry of {K2(H2O@Q[5])}[InCl4 · 2H2O]Cl · 4.5H2O. Contained within the crystal structure is a repeating pattern of interlinked Q[5] “beads” with eight “beads” per ring and each ring interlinked and stacked as depicted in Figure 2b,d. There are two potassium ions K1 and K2 that form the basis for the interlinking ring structure between the Q[5]’s by direct coordination to the carbonyl O’s of the portals and then a shared coordination to water molecules (Figure 2). Essentially there are eight “beads” of Q[5] threaded on a repeating molecular string of -K2-O5W-K2-(O7W-O4W)-K1-O2W-K1-(O4W-O7W)K2- where O4W and O7W are included water within the cavity of Q[5] with a 50% occupancy for each (i.e., O4W and O7W represent one water molecule in the cavity). The apparent corners of the 8-membered Q[5] “beaded” rings are also branch points to another and equal fused 8-membered ring, which means that O2W is repeated (K1 coordinates to two water molecules labeled as O2W). The branch is the corner of the next fused ring (Figure 2b, corners are the same as the star bonding pattern at the center of Figure 2b, also see Supporting Information). This provides an interesting example of solidstate supramolecular ring formation using Q[5] as a building block, where the rings are apparently induced by the large anion InCl4-.25 In contrast, the crystals obtained from R,R′-DMeQ[5] and SrCl2 or TriCyHQ[5] and KCl also form supramolecular structures with the substituted Q[5] as “beads” arranged in rings. However, these rings are formed by direct interconnection by the metal ions Sr2+ and K+ respectively and the Q[5] portals. R,R′-DMeQ[5]SrCl2 crystals have a stoichiometry of {Sr2(Cl@ R,R′-DMeQ[5]}3Cl · 19H2O. The most interesting structure found within these crystals was that six of the R,R′-DMeQ[5] “beads” were arranged in a ring (Figure 3). There are two noteworthy features in this structure. First, the methyl substit-

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Figure 3. The 6-membered R,R′-DMeQ[5] “beaded” ring, crystal structural arrangement. Hydrogen atoms, solvate water molecules are omitted for clarity. Cl- green, Sr2+ pink/purple, Me groups yellow, and O red.

uents do not all point to a hydrophobic core at the center of the 6 “beaded” ring but rather, the Me groups of every alternate R,R′-DMeQ[5] point to the center and the remainder to the outside. Second, the coordinating metal ions, Sr2+ (Sr3, Sr4, Sr7, Sr8, Sr11 and Sr12) are coordinated to the portals of the 3 R,R′-DMeQ[5] where their Me substituents point to the outside of the 6 “beaded” molecular bracelet and are also directly linked to one of the carbonyl oxygens of the adjacent R,R′-DMeQ[5] portal. This occurs specifically as a coordinating link between each of the carbonyl O’s (O6, O21, O26, O41, O46 and O1 respectively) of the Me substituted glycoluril moiety at each portal directly to the adjacent portal Sr2+ of the neighboring Q. Adjacent R,R′-DMeQ[5] portals are offset to each other and the faces of these portals are at an angle of 58° (av.). It is the direct and specific coordination to the Me glycoluril moiety carbonyl oxygen that provides the short link between adjacent R,R′-DMeQ[5] portals and draws the edge of one portal nearer the center of the other. This provides the junction angle to form the 6-membered R,R′-DMeQ[5] “beaded” ring. The larger angled opening (“bead” junctions at the outer ring surface) of the R,R′-DMeQ[5] portal face interactions is filled by six pairs of shared water molecules. In addition, Sr1, Sr2, Sr5, Sr6, Sr9 and Sr10 are coordinated to six separate water molecules (O1W, O4W, O9W, O10W, O15W, and O16W, respectively). This then completes the Sr2+ ions with a coordination of eight. Each Q in the molecular bracelet has encapsulated a single Cl- ion consistent with unsubstituted Q[5] reports.11,12 Not all of the R,R′-DMeQ[5] are arranged in “beaded” rings. There are R,R′-DMeQ[5] coordinated pairs and some lone R,R′DMeQ[5]. The pairs are linked in the same fashion as a pair in the ‘beaded’ ring except that the ends of the pairs are terminated with two water molecules coordinated to the terminal portal Sr2+ ion completing a coordination of seven. The linking Sr2+ ions of the two different pair types (Sr14 to Sr15 and Sr19 to Sr18) are coordinated with eight O just as occurs in the “beaded” ring. The single R,R′-DMeQ[5] coordinated to Sr2+ ions (Sr21-Sr22 and Sr23-Sr24) have a coordination of seven, five portal O each and two water molecules. Each of the three forms,

Ni et al.

Figure 4. (a) The stacking of the 6-membered R,R′-DMeQ[5] “beaded” rings, the “beaded” pairs and single “beads” arranged in the crystal lattice, top view. (b) Side view. Hydrogen atoms, solvate water molecules and anions are omitted for clarity.

the 6-membered “beaded” rings (molecular bracelet), the linked pairs or the lone DMeQ[5], are arranged in the crystal lattice where the bracelets are stacked in columns and the intermolecular space between the columns is filled with linked pairs and single R,R′-DMeQ[5] (Figure 4). At the center of the 6-membered “beaded” ring is a molecular cavity with, shortest and longest dimensions of ∼6.5 × ∼8.9 Å (between carbon centers Me and methine directly opposite) and ∼8.5 Å deep. The interior surface of this cavity is constructed of the methine carbon outer surfaces of three of the R,R′-DMeQ[5] and six methyl groups of three of the remaining R,R′-DMeQ[5]. The cavity contains four free water molecules (O192, O195, O202 and O211 see Supporting Information). The 6-membered “beaded” rings are stacked upon one another to give a channel with a cross section resembling a 6-pointed star. The intermolecular space between the molecular bracelets contains additional water, but directly in line with the channel is water (O184) and Cl- ions (Cl37 and Cl41). Clions are not found in the cavity. Each R,R′-DMeQ[5] no matter what structural arrangement has encapsulated a Cl- ion. The third example of supramolecular ring formation was found for the R,β,δ-TriCyHQ[5] in the presence of KCl in water. Crystals obtained from this mixture had a stoichiometry of {K2(H2O@R,β,δ-TriCyHQ[5])}2Cl · 15.5H2O. The X-ray crystal structure showed in the simplest description was a puckered 10-membered “beaded” ring with each “bead” (R,β,δ-TriCyHQ[5]) linked by K+ ions. In reality it is much more complex, with each of the junctions linked by K+ ions forming trigonal planar branches, to fused 10-membered rings. In addition, there are two separate multifused 10-membered ring network structures that are independent of each other, and each of these networks fills the spaces created in the separate but equivalent network (Figure 6a-d). The two networks are catenated and are not linked but are spaced by water molecules and Cl- ions. The K+ ions (K1 and K2) are coordinated to eight

Alkyl-substituted Cucurbit[5]uril in Rings

Figure 5. The crystal structure of the 10-membered R,β,δ-TriCyHQ[5] “beaded” ring. (a) TriCyHQ[5] with K+ ions K1 and K2 showing their coordinating atoms. (b) The TriCyHQ[5] shown in its 10-membered ring arrangement with each “bead” a repeat of that shown in a. (c) The branch junction to the fused 10-membered rings.

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neighbor carbonyl oxygen portals and two water molecules (Figure 5a). The coordination of a K+ ion to the portal carbonyl oxygen atoms and a direct coordination to a near neighbor portal carbonyl oxygen atom draws one edge of each portal close together, which forms the angle at each junction to create the ring. Each junction or node has three K+ ions all coordinated in a similar way (Figure 5c and Supporting Information). As in the previous example, it is the direct coordination to the carbonyl oxygen atom of the substituted glycoluril moiety that favors the formation of junction angles (Figure 5a,c). The K+ ions K1 and K2 are coordinated, respectively, to the portal carbonyl, O1-O5 and O6-O10. K1 and K2 are also directly coordinated to O8 and O1 respectively of the neighboring portal (Figure 5a). The size of the ring (10-membered “beaded” ring) appears to be dictated by the overall diameter of the R,β,δ-TriCyHQ[5] which measures ∼12.6 Å from one cyclohexane ring to the other, opposite in the ring. The strings of R,β,δ-TriCyHQ[5] “beads” which make up the 10-membered rings, pass through each ring and the rings have an average diameter of ∼19.3 Å appropriate to fit the diameter of R,β,δ-TriCyHQ[5]. The observed solid state affinity of the metal ion in both examples, R,R′-DMeQ[5] and R,β,δ-TriCyHQ[5], for the carbonyl oxygen atom of the substituted glycoluril moiety, indicates a likely increased electron density as a consequence of the electron donating effect of the alkyl substituents. An alternative explanation, such as a change in oxygen bond distances compared to the free ligand or portal distortion, were not evident. It is this affinity in the case of substituted Q[5] that favors the formation of “beaded” rings. The formation of rings creates the possibility to form cavities. The 6-membered “beaded” ring of R,R′-DMeQ[5] has a cavity of ∼6.5 × ∼8.5 × ∼8.5Å, filled only with free water, while the 10-membered “beaded” ring of R,β,δ-TriCyHQ[5] of ∼19.3 Å diameter is filled by a string of R,β,δ-TriCyHQ[5]. In both cases, the potential to replace the contents of the rings with other molecules is inspiring. The 6-membered “beaded” ring of R,R′-DMeQ[5] is of particular interest in this regard as the water molecules are free in a relatively nonpolar environment created by the methine and methyl cavity walls. Conclusion We have demonstrated three novel ringed supramolecular structures from three different Q[5]’s. The first example was a large anion-induced 8-membered ring structure derived from Q[5], while the other two were substituent induced 6- and 10membered ring structures. Q[5]’s as single units may have limited application due to their small cavities, but as supramolecular building blocks the possibilities appear greater, especially for substituted Q[5]. The metal coordination selectivity for the oxygen carbonyls of the alkyl-substituted glycoluril moieties is being examined further to ascertain the predictability of ring formation relative to substitution.

Figure 6. The crystal structural arrangement of the 10-membered “beaded” rings. (a-b) Interlocking of 10-membered rings. (c) The interrelationship of interlocking 10-membered ring networks. (d) A representation of the fused 10-membered rings and their interlocking relationship. Hydrogen atoms, solvate water molecules, and anions are omitted for clarity.

oxygen atoms, the five carbonyl oxygen atoms of a R,β,δTriCyHQ[5] portal, direct coordination to two adjacent near

Acknowledgment. We are gratefully to the Natural Science Foundation of China (No. 20662003, 20767001), International Collaborative Project of Guizhou Province, and the Governor Foundation of Guizhou Province for financial support. Support is also from the Australian Defence Force Academy for the StartUp-Grant. Supporting Information Available: Synthetic information and additional structural details are available free of charge via the Internet at http://pubs.acs.org.

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Ni et al.

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Tseng, H.-R.; Stoddart, J. F.; Heath, J. R. Nature 2007, 445, 414– 417. Griffiths, K. E.; Stoddart, J. F. Pure Appl. Chem. 2008, 80, 485–506. Passaniti, P.; Ceroni, P.; Balzani, V.; Lukin, O.; Yoneva, A.; Vogtle, F. Chem. Eur. J. 2006, 12, 5685–5690. Sun, S.; Zhang, R.; Anderson, S.; Pan, J.; Akermark, B.; Sun, L. Chem. Commun. 2006, 4195–4197. Kang, J.-K.; Hwang, I.; Young, H.; Jeon, W. S.; Kim, H.-J.; Kim, K. Supramol. Chem. 2008, 20, 149–155. Gerasko, O. A.; Samsonenko, D. G.; Fedin, V. P. Russ. Chem. ReV. 2002, 71, 741–760. Lim, S.; Kim, H.; Selvapalam, N.; Kim, K.-J.; Cho, S. J.; Seo, G.; Kim, K. Angew. Chem., Int. Ed. 2008, 47, 1–5,, DOI: 10.1002/ anie.200800772. Hasenknopf, B.; Lehn, J.-M.; Boumediene, N.; Dupont-Gervias, A.; Van Dorsselaer, A.; Kneilsel, B.; Fenske, D. J. J. Am. Chem. Soc. 1997, 119, 10956–10962. The crystal data for each compound was collected on a Bruker Apex2000 CCD diffractometer using graphite monochromated Mo KR radiation (λ ) 0.71073 Å) with ω scan mode. Data were collected at 223 K. Structural solution and full matrix least-squares refinement based on F2 were performed with the SHELXS-97 and SHELXL-97 program package respectively. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were generated geometrically. (a) {K2(H2O@Q[5])}[InCl4 · 2H2O]Cl · 4.5H2O: orthorhombic, a ) 27.4149(2) Å, b ) 27.4149(2) Å, c ) 13.3062(2) Å, V) 10000.63(18) Å3, Z ) 4, Dcalcd ) 1.775 g cm-3, µ ) 0.996 mm-1, unique reflns ) 4368, obsd reflns ) 3338, params ) 367, R [I > 2σ(I)]a ) 0.0619, wR [I > 2σ(I)]b ) 0.2053. (b) {Sr2(Cl@R,R′DMeQ[5]}3Cl · 19H2O: triclinic, a ) 27.7887(19), b ) 35.544(3), c ) 36.548(3) Å, R ) 60.922(3)°, β ) 84.036(4)°, γ ) 89.995(4)°, V) 31325(4) Å3, Z ) 384, Dcalcd ) 2.659 g cm-3, µ ) 16.260 mm-1, unique reflns ) 141413, obsd reflns ) 50202, params ) 3865, R [I > 2σ(I)]a ) 0.1311, wR [I > 2σ(I)]b ) 0.3324. (c) {K2(H2O@R,β,δTriCyHQ[5])}2Cl · 15.5H2O: cubic, a ) 33.3373(4) Å, b ) 33.3373(4) Å, c ) 33.3373(4) Å, V ) 37050.3(8) Å3, Z ) 24, Dcalcd ) 1.510 g cm-3, µ ) 0.338 mm-1, unique reflns ) 10877, obsd reflns ) 5407, params ) 850, R [I > 2σ(I)]a ) 0.1154, wR [I > 2σ(I)]b ) 0.3374.

CG800451Z