Effects of Cocrystalline Subunits on the Supramolecular Chemistry of

Feb 4, 2011 - structures based on the coordination of Me10Q[5] with alkali metals ... the coordinated aqua ligands located on the supramolecular chain...
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DOI: 10.1021/cg101355d

Effects of Cocrystalline Subunits on the Supramolecular Chemistry of Me10Q[5]: From Simple Inorganic Anions to Cluster Anions

2011, Vol. 11 778–783

Jingxiang Lin, Jian L€ u,* Minna Cao, and Rong Cao* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou 350002, China Received October 13, 2010; Revised Manuscript Received December 21, 2010

ABSTRACT: Five supramolecular assemblies, formulated as {K2(H2O)3 3 [(Me10Q[5])@(H2O)0.5]} 3 Cl2 1, [K2(H2O)2 3 (Me10Q[5]@H2O)](ClO4)2 3 H2O 2, {K(H2O) 3 K(H2O)(SCN)Me10Q[5]} 3 (SCN) 3 H2O 3, [Na2(H2O)2K2(H2O)4(Me10Q[5])] 3 [Mo8O26] 4 and [Na2(H2O)2Rb2(H2O)4(Me10Q[5])] 3 [Mo8O26] 5, have been successfully isolated. Compounds 1-5 exhibit various structures based on the coordination of Me10Q[5] with alkali metals controlled by a series of anions, in which the anions play multiple roles of coordination, space-filling, charge-compensating etc. In the structure of compound 1, Me10Q[5] is capped by two [K(H2O)2]þ moieties through the carbonyl O atoms on the portals to form a bicapped unit. The bicapped Me10Q[5] units are interconnected to form a covalent bonded chain structure. Uncoordinated Cl- anions fill in the crystal lattice behaving as both counterions and space-filling agents. Compound 2 possesses isolated bicapped units comprised by Me10Q[5] and [K(H2O)]þ moieties, which are further linked via extensive hydrogen bonding interactions between the coordinated and uncoordinated water molecules into a supramolecular chain structure. The ClO4- anions are involved in the formation of hydrogen bonds with the coordinated aqua ligands located on the supramolecular chains. Compound 3 features an asymmetric bicapped Me10Q[5] unit due to the coordination of a [K(H2O)]þ or [K(H2O)(SCN)]þ on each portal of Me10Q[5]. The bicapped units in 3 are further extended into a supramolecular chain structure by hydrogen bonds between the coordinated water and carbonyl O atoms of Me10Q[5]. Moreover, uncoordinated SCN- anions are also observed in the crystal lattice. Compounds 4 and 5 are isomorphous and display interesting 3D network structures built by the interconnections of Me10Q[5], mixed alkali metal cations (Naþ/Kþ for 4 and Naþ/Rbþ for 5), and [R-Mo8O26]4- cluster anions. In this study, the supramolecular assemblies of Me10Q[5] based solids show an anion dependent feature, which is systematically explored.

Introduction Supramolecular solids built by large subunits have long been paid great research interest from chemists and material scientists because of not only the valuable crystallographic information contributing to supramolecular chemistry but also the potential applications in molecular separation, functional materials, molecular devices and new nanotechnology.1 In recent years, pioneering studies have been focused on the cucurbit[n]uril family (denoted as Q[n], see Figure 1a),2 which represents a class of new macrocyclic cavitand featuring barrel-like shape, hydrophobic capacity, and two carbonyl fringed portals. The preparation of Q[n] homologues, including Q[5], Q[6], Q[7], Q[8], and a series of Q[n]s analogues2-4 defines a booming research target toward the rational design of supramolecular self-assemblies using well-defined organic entities. Investigation in the supramolecular chemistry of Q[n]s with metal- or cluster-aqua complex counterparts was mainly focused on those with Q[6] or Q[8] as building units.2 Recently, research attention has been paid to a wider range of cucurbit[n]urils homologues and derivatives such as Q[5], Q[7] and Me10Q[5].3 Due to the strong interactions between carbonyl oxygen donor atoms and cations, stable complexes form with metal-aqua complexes covering each portal of the cucurbituril, like a “lid” on a “barrel”, and the Q[n]s acting as multidonor organic ligands. As a consequence, a number of adducts of cucurbituril and metal ions and clusters have been prepared.3,5-10 Coordination polymers comprising discrete molecular capsules or one-dimensional chain structures based on such molecular moieties were extensively synthesized. Very recently, a novel crystalline supramolecular architectural pubs.acs.org/crystal

Published on Web 02/04/2011

Figure 1. Structural presentation of Q[n]s (a) and Me10Q[5] (b).

system was constructed, in which Q[5]s and Kþ ions form infinite chains through coordination and ion-dipole interaction, and form a series of wavelike “walls”, with hydroxybenzene or its derivatives and the counteranions included in the crystal spaces.11 Studies on the scope of Me10Q[5] (decamethylcucurbit[5]uril) (Figure1), an important member of Q[n]’s family first described in 1982 and its basic structure found by X-ray analysis a decade thereafter,12,13 as a ligand for supramolecular cocrystalline assemblies are limited. Besides its unique rigid structure, Me10Q[5] possesses an easy preparation method, reasonable solubility, and chemical and thermal stability, which make it very attractive for coordination of cations in aqueous solutions.14,15 Due to the strong interactions between carbonyl oxygen donor atoms and cations, like its unsubstituted Q[n] analogue, Me10Q[5] can also form stable complexes with metal cations, in which the metal ions and their coordinated water molecules cover each portal of Me10Q[5] like a ‘‘lid’’ on a ‘‘barrel’’.3f,j In the previous study, the coordination effect of cations on the portal of the Q[n] family was speculated as a main reason for increasing the solubility of Q[n]s in r 2011 American Chemical Society

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water,7 which we believed to play a comparable role in the crystallization of the Me10Q[5]-based solids. On the other hand, anions play extraordinary roles in environmental, chemical, and biological processes, and thus anion bonding or templating is always a major issue of research from both an academic and practical point of view.16 In general, anions are able to act as charge-compensating, spacefilling and, sometimes, templating agents throughout the cocrystalline process of supramolecular assemblies. Anions can also coordinate effectively to metal centers as auxiliary ligands preventing or templating the further coordination of primary ligands17-22 and have been recently known to have significant influence on crystal packing.23 In the realm of supramolecular solids based on Q[n]s, anions are also known to have subtle influence on the structures of the complexes. Simple anions, Cland NO3-, were found serendipitously to be involved in the crystal packing in the supramolecular complexes constructed from Q[n]s and a wide range of metal ions2a,3c,3h,6 with diverse supramolecular structures. Thuery investigated the inclusion properties of Q[n]-based solids encapsulating different anions of ReO4- and NO3- which resulted in the different coordination pattern of lanthanides with CB[n]. The anions were believed to influence the final structures of the complexes in addition to the primary interplay between metals and CB[n] units.3d,l These examples proved that anions may play subtle and crucial roles in the supramolecular assemblies of Q[n]s, which remain yet to be uncovered. Herein, we present the investigation on the effects of anions in crystal packing with a series of supramolecular solids based on Me10Q[5]. Various anions including simple inorganic anions and cluster anions have been used to assemble with Me10Q[5] in the presence of appropriate alkali metal cations. Five supramolecular assemblies based on Me10Q[5], formulated as {K2(H2O)3 3 [(Me10Q[5])@(H2O)0.5]} 3 Cl2 1, [K2(H2O)2 3 (Me10Q[5]@H2O)](ClO4)2 3 H2O 2, {K(H2O) 3 K(H2O)(SCN)Me10Q[5]} 3 (SCN) 3 H2O 3, [Na2(H2O)2K2(H2O)4(Me10Q[5])] 3 [Mo8O26] 4 and [Na2(H2O)2Rb2(H2O)4(Me10Q[5])] 3 [Mo8O26] 5, were reported. The results indicate an anion dependent feature in the supramolecular assemblies of Me10Q[5] based solid materials. Experimental Section General. Me10Q[5] was synthesized according to the literature,11 and other chemical reagents were of commercial origins and used without further purification. The hydrothermal reactions were performed in 25 mL Teflon-lined stainless-steel vessels under autogenous pressure with a filling capacity of approximate 80%. Elemental analyses (C, H, and N) were carried out on an Elementar Vario EL III analyzer. X-ray powder diffractions (XRPD) were performed with a RigakuDMAX2500 diffractometer. Synthesis of {K2(H2O)3 3 [(Me10Q[5])@(H2O)0.5]} 3 2Cl (1). Potassium chloride (0.074 g, 1.0 mmol) and Me10Q[5] (0.075 g, 0.077 mmol) were dissolved in 20 mL of distilled water. The solution was stirred until all reactants were dissolved. Then the solution was allowed to evaporate slowly in air. Colorless crystals suitable for single-crystal X-ray diffractions formed within several days in a yield of approximately 75% (based on Me10Q[5]). Anal. Calcd for K2Cl2C40H57 N20 O13.5 (M = 1183.1): C, 40.57; H, 4.81; N, 23.67. Found: C, 40.54; H, 5.12, N, 23.64. Synthesis of [K2(H2O)2 3 (Me10Q[5]@H2O)](ClO4)2 3 H2O (2). To an aqueous solution of Me10Q[5] (10 mL, 0.003 mol 3 L-1) was added KCl (0.030 g, 0.40 mmol). The solution was stirred thoroughly and transferred to a test tube, and then 10 mL of aqueous solution of NaClO4 3 H2O (0.105 g, 0.75 mmol) was gently added to the test tube. The solution was allowed to diffuse at room temperature. Hours later, well-shaped colorless crystals of 2 were obtained in a yield of approximately 82% (based on Me10Q[5]). Anal. Calcd for

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K2Cl2C40H58O22N20 (M = 1320.1): C, 36.37; H, 4.39; N, 21.21; Found: C, 36.42; H, 4.38; N, 21.78. Synthesis of {K(H2O) 3 K(H2O)(SCN)(Me10Q[5])} 3 (SCN) 3 H2O (3). An aqueous solution of Me10Q[5] (10 mL, 0.003 mol 3 L-1) was added to a test tube. To this tube potassium thiocyanate (0.048 g, 0.5 mmol) in 10 mL of deionized water was gently added to the top of the Me10Q[5] solution. The solution was allowed to diffuse at room temperature. Hours later, well-shaped colorless crystals of 3 were obtained in a yield of approximately 86% (based on Me10Q[5]). Anal. Calcd for K2C42H56N22O13S2 (M = 1219.4): C, 41.34; H, 4.59; N, 25.26; Found: C, 41.39; H, 4.45; N, 25.93. Synthesis of [Na2(H2O)2K2(H2O)4(Me10Q[5])] 3 [Mo8O26] (4). Na2MO4 3 2H2O (0.30 g, 1.24 mmol), KCl (0.083 g, 1.1 mmol) and Me10Q[5] (0.075 g, 0.077 mmol) were dissolved in 15 mL of water. pH of the solution was adjusted to 2.0-3.0 by adding hydrochloric acid, and the solution was stirred for another hour. The above solution was then sealed in Teflon-lined stainless-steel vessels under autogenous pressure at 140 °C for 3 days and cooled to room temperature within a day. Colorless crystals of 4 were separated and dried on a filter paper. Yield: 57% (based on Me10Q[5]). Anal. Calcd for Na2K2Mo8C40H62N20O42 (M = 2386.7): C, 20.11; H, 2.59; N, 11.73; Found: C, 20.21; H, 2.62; N, 11.53. Synthesis of [Na2(H2O)2Rb2(H2O)4(Me10Q[5])] 3 [Mo8O26] (5). The general procedure was similar to the preparation of compound 4. Na2MO4 3 2H2O (0.30 g, 1.24 mmol), rubidium nitrate (0.147 g, 1.0 mmol) and Me10Q[5] (0.075 g, 0.077 mmol) were dissolved in 15 mL of water. The pH of the solution was adjusted to 3.0-4.5 with hydrochloric acid. The solution was then stirred for 1 h and sealed in Teflonlined stainless-steel vessels under autogenous pressure at 140 °C for 3 days and cooled to room temperature within a day. Colorless crystals of 5 were isolated and dried on a filter paper. Yield: 63% (based on Me10Q[5]). Anal. Calcd for Na2Rb2Mo8C40H62N20O42 (M = 2479.5): C, 19.36; H, 2.50; N, 11.29; Found: C, 19.52; H, 2.53; N, 11.65. X-ray Crystallography. Diffraction intensities for 1-5 were collected on an Oxford Xcalibur, Eos diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073). Multiscan absorption corrections were performed with the CrysAlisPro program (Oxford Diffraction Ltd., Version 1.171.33.66). Empirical absorption correction was carried out using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Structures were solved by direct methods and refined on F2 by full-matrix least-squares with the SHELXTL-97 program package.24 All non-hydrogen atoms, except some water oxygen atoms, were refined anisotropically. The hydrogen atoms of organic molecules were generated geometrically. Restraints on displacement parameters were applied for some solvent water molecules. Some hydrogen atoms bound to water oxygen atoms were found on a Fourier- difference map. Special details are as follows: The hydrogen atoms bound to water oxygen atoms were not found in compound 1. Solvent water oxygen atoms O1w and O3w were given occupancy factors of 0.5 and 0.25, respectively, and refined isotropically, in order to retain acceptable displacement parameters. Hydrogen atoms bound to O1w, O2w and O3w were not found on a Fourier-difference map. For compound 2, solvent water oxygen atoms O2w and O3w were given occupancy factors of 0.25 and 0.5, respectively, and refined isotropically, in order to retain acceptable displacement parameters. In compound 2, all the hydrogen atoms bound to water oxygen atoms can be found on Fourier-difference map. For compounds 3-5, no hydrogen atoms bound to water oxygen atoms were found on a Fourier-difference map. A summary of the crystallographic data and refinement parameters for compounds 1-5 is given in Table 1.

Results and Discussion The X-ray crystal structure of 1 reveals that each Me10Q[5] is bicapped by two potassium ions on both of its portals with a halfoccupied water molecule encapsulated in its cavity (Figure 2a). The crystallographically independent potassium ion is coordinated by five carbonyl O atoms of the portal with K-O distances ranging from 2.660 A˚ to 2.949 A˚, resulting in the formation of a doubly charged molecular capsule {K2(H2O)4 3 [(Me10Q[5])@0.5H2O]}2þ. The molecular capsules are connected together by sharing the μ-2 water molecule (O1w) leading to the

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Table 1. Crystal Data and Structure Refinement for Compounds 1-5 compounds

1

2

3

empirical formula K2Cl2C40H57N20O13.5 K2Cl2C40H58O22N20 K2C42H56N22O13S2 formula wt 1183.1 1320.1 1219.4 temp (K) 293(2) wavelength (A˚) 0.71073 cryst syst monoclinic space group C2/c C2/c P21/c a (A˚) 24.4141(9) 24.5971(1) 22.222(4) b (A˚) 17.7598(4) 17.9071(7) 18.009(4) c(A˚) 12.7352(4) 13.1020(7) 12.897(3) β(deg) 116.051(4) 115.756(7) 97.08(3) 3 4960.8(3) 5197.6(4) 5121.9(2) vol (A˚ ) Z 4 2 4 0.917 1.036 goodness-of-fit on F2 1.091 a 0.0581 0.0882 final R indices R1 = 0.0660 0.1379 0.1747 [I > 2 < σ(I)] wR2b = 0.2005 P P P P a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2.

4

5

Na2K2Mo8C40H62N20O42 2386.7

Na2Rb2Mo8C40H62N20O42 2479.5

C2/c 17.271(3) 14.611(3) 28.685(6) 101.700(4) 7088(2) 4 1.168 0.0692 0.2000

C2/c 17.323(4) 14.597(3) 28.741(7) 100.931(4) 7135(3) 4 1.158 0.0623 0.1581

Figure 2. X-ray crystal structures of compounds 1 (a), 2 (b), 3 (c), 4 and 5 (isomorphic, d) in ORTEP drawing with thermal ellipsoids at 30% probability.

formation of a cationic one-dimensional chain subunit Figure 3. The Cl- behaves as counteranions in the crystal lattice by involving in halogen bonding interactions (Cl1 3 3 3 O2w 3.285 A˚), leading to a 3D supramolecular structure. Next, we hoped to replace the Cl- anions in the solution of Me10Q[5] and potassium ion with ClO4- ions, which has a

tetrahedral geometry, to investigate the packing pattern in the solid state. The attempts to obtain an X-ray quality single crystal with the method used for compound 1 were unsuccessful because of the fast precipitation from the mixture of Me10Q[5] and KClO4 aqueous solution. Compound 2 was luckily obtained by the conventional diffusion method. What is worth

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Figure 3. View of the one-dimensional chain subunit in compound 1.

Figure 4. View of the H-bonded chain unit in compound 2.

Figure 5. View of the H-bonded chain unit in compound 3.

mentioning is that NaClO4 was used in the reaction due to the sparing solubility of KClO4 in water. Single crystal X-ray diffraction shows that a similar molecular capsule {K2(H2O)2 3 [(Me10Q[5])@H2O]}2þ has been observed in the structure of compound 2 (Figure 2b). The molecular capsules link together through hydrogen bonds formed between the uncoordinated and coordinated water molecules (O1wA 3 3 3 O3w bond length 2.934 A˚) to form a 1D supramolecular chain subunit (Figure 4). Moreover, the ClO4- behave as counteranions in the crystal lattice and anchor to each side of the supramolecular chain via strong hydrogen bonding O1w 3 3 3 O8, with a bond length of 2.951 A˚. The chains are further extended into a 3D supramolecular structure through weak H-bonds formed by aqua ligand (O1w) and O donor of ClO4- with O1w 3 3 3 O6 distance of 3.168 A˚. In the synthesis of compound 3, KSCN was used as an anion source. X-ray crystallography shows that SCN- anions have been successfully incorporated into the structure of 3. Moreover, the SCN- anions act not only as counterions and space-filling species but also as ligands coordinating to potassium centers. The crystallographically independent K(1) atom is seven-coordinated by five carbonyl O donors, one aqua ligand, and an SCN- anion, while the K(2) is sixcoordinated by five carbonyl O donors and one aqua ligand (Figure 2c). Thus, an asymmetric bicapped molecular capsule {K2(H2O)(SCN) 3 [(Me10Q[5])@H2O]}þ with an end-on decoration of SCN- is formed in compound 3. Moreover, an extra uncoordinated SCN- anion exists in the crystal lattice. The coordinated water molecules (O1w and its equivalents) from the molecular capsules take part in hydrogen bonds (bond distance 2.958 A˚) with portal oxygen atoms (O6) of

Me10Q[5], leading to the formation of a H-bonded supramolecular chain. Additionally, uncoordinated counteranions, SCN-, involve in strong hydrogen bonding interactions with aqua ligands (O2w 3 3 3 N1 2.800 A˚) from the supramolecular chain (Figure 5), leading to the formation of a netural 1D subunit. The chain subunits are further extended to a 3D structure through supramolecular interactions. In order to exam the effects of large cluster anions on the crystal packing, we introduce purposefully polymolybdate anions into the reaction system. Simple Na2MoO4 3 2H2O salt was used, and the self-assembly of polymolybdate anions was expected. Hydrothermal reaction of aqueous solution of Na2MoO4 3 2H2O, KCl/RbNO3 and Me10Q[5] gave compounds 4 and 5, respectively. 4 and 5 are isomorphous; thus only the structural details of compound 4 are presented. Compound 4 crystallizes in monoclinic space group C2/c. X-ray crystallography reveals that the structure of 4 is built by R-{Mo8O26}4clusters, bicapped molecular capsules {K2(H2O)4 3 [(Me10Q[5])@H2O]}2þ, and hydrated sodium cations (Figure 2d). The wellknown R-{Mo8O26}4- cluster is centrosymmetric and composed of a ring of six edge-sharing {MoO6} octahedra capped by two {MoO4} tetrahedra above and below the central ring (Figure 6) with Mo-O distances in the range of 1.682 A˚ to 2.522 A˚ and bond angles in the range of 78.62° to 133.21° (Table S2 in the Supporting Information); the average Mo-O distance of MoO4 tetrahedra (1.766 A˚), comparable to that of Na2MoO4 3 2H2O (1.772 A˚),25 is shorter than that of MoO6 octahedra (2.01 A˚). The bicapped molecular capsule in 4 is very similar to that of 1. The R-{Mo8O26}4- clusters serve as inorganic ligands linking the molecular capsules into a 1D chain subunit (Figure 7a). The Me10Q[5] moieties are interconnected by

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potassium cations, sodium cations and octamolybdate anions leading to the formation of a 3D network (Figure 7b). Aggregation of the molecular capsules in compounds 1-5 was generally tuned by the anions present in the reaction systems. There are two major reasons contributing to the formation of the molecular assemblies. One is the coordination affinity of Me10Q[5] to alkali metals. The carbonyl groups can bind to alkali metal ions effectively leading to the formation of molecular capsules. The other is the cocrystalline effect of anionic species. The geometry and coordination ability of the anions play crucial roles in the formation of various molecular assemblies. We come to a conclusion from the structural analysis that, although the molecular capsules in compounds 1-3 are nearly identical, the basic chain subunits display significant variation according to the anionic spaces Cl-, ClO4- and SCN-. Due to the different size, shape and coordination ability of the counterions, the cationic molecular capsules assemble into a covalently bonded chain (compound 1), H-bonded chain joined by water molecules (compound 2 and 3), and interesting 3D network structures in compounds 4 and 5. Furthermore, the different crystallization rates of these compounds also indicate the anion

Figure 6. Structural representations of the octa-molybdenumoxide anion [R-Mo8O26]4- in compounds 4 and 5.

Lin et al.

effects on crystallization processes. To be specific, the simple anion Cl- in compound 1 participates in weak halogen bonding with the basic chain subunits, resulting in a slow crystallization, while bulky and multiatomic anions are more intent to form strong and multiple hydrogen bonds with the chain subunits, leading to the fast crystallization of compounds 2 and 3. It should be mentioned that the number of charges of the molecular capsules may play a subtle role in the crystallization process of the Me10Q[5] based molecular entities. That is, the formation of doubly charged molecular capsules of {K2(H2O)2 3 [(Me10Q[5])@0.5H2O]}2þin compound 1 contributes to the slow crystallization in obtaining the compounds, whereas the univalent {K2(H2O)2(SCN) 3 [(Me10Q[5])@H2O]}þ molecular capsule in compound 3 favors a fast crystallization. We believe the different aggregation pattern predestined the diverse synthetic method of single crystals and the methodology can be applied in related reaction systems. Conclusion In conclusion, we report the effects of anions in the supramolecular assemblies based on Me10Q[5] with alkali metal cations. Five supramolecular compounds have been successfully synthesized. Compounds 1-5 exhibit interesting structures based on the coordination of Me10Q[5] with alkali metals controlled by various anions, in which the anions are observed to play multiple roles of coordination, space-filling, charge-compensating etc. In the structures of compounds 1-5, Me10Q[5]s assemble with alkali metal cations Kþ (for 1-4) or Rbþ (for 5) to form the bicapped molecular capsule units. The basic molecular capsules are interconnected by supramolecular or covalent forces into various structures showing interesting anion dependent features. In the structure of compound 1, the bicapped Me10Q[5] units are interconnected to form covalent bonded chain subunits. Compound 2 possesses isolated bicapped units linking through H-bonds into supramolecular chain subunits. Compound 3 features a

Figure 7. View of the one-dimensional chain subunit in compounds 4 and 5 (a); the 3D framework structure of compounds 4 and 5 (b) (K for compound 4, Rb for compound 5).

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supramolecular chain subunit built by asymmetric bicapped molecular capsules jointed via self-compliment H-bonds. Compounds 4 and 5 are isomorphous and display interesting 3D network structures built by the interconnections of Me10Q[5], mixed alkali metal cations (Naþ/Kþ for 4 and Naþ/ Rbþ for 5), and [R-Mo8O26]4- cluster anions. Acknowledgment. We are grateful for financial support from 973 Program (2011CB932504, 2007CB815303), NSFC (20731005, 20821061, 20873151), Fujian Key Laboratory of Nanomaterials (2006L2005), and Key projects from CAS. Supporting Information Available: A summary of coordinated bonds in compounds 1-5, Mo-O distances of R-{Mo8O26}4- in compounds 4 and 5, experimental and simulated XRPD spectra of compounds 1-5, and crystallographic information files (CIF format) of crystals 1-5. This material is available free of charge via the Internet at http://pubs.acs.org.

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