Construction of Train-Like Supramolecular Structures from

Feb 19, 2010 - In the molecular structures of the three compounds, versatile Keggin-type polyoxotungstate clusters were successfully assembled as inor...
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DOI: 10.1021/cg1000528

Construction of Train-Like Supramolecular Structures from Decamethylcucurbit[5]uril and Iso- or Hetero-Keggin-Type Polyoxotungstates

2010, Vol. 10 1966–1970

Jing-Xiang Lin, Jian L€ u, Hong-Xun Yang, 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, P. R. China Received January 15, 2010

ABSTRACT: A family of inorganic-organic hybrid solid materials has been assembled from presynthesized organic decamethylcucurbit[5]uril (Me10Q[5]) macrocycles and the well-known inorganic polyoxometalate (POM) clusters. Three compounds, formulated as [Na6(H2O)11(Me10Q[5])2] 3 [β-H2W12O40] 3 5H2O (1), [Na4(H2O)7(Me10Q[5])2] 3 [R-SiW12O40] 3 20H2O (2), and [Na4(H2O)7(Me10Q[5])2] 3 [β-SiW12O40] 3 13H2O (3) are reported. In the molecular structures of the three compounds, versatile Keggin-type polyoxotungstate clusters were successfully assembled as inorganic building blocks to build hybrid materials with Me10Q[5]. Compound 1 comprises β-Keggin-type iso-polyoxotungstate [H2W12O40]6- clusters and Me10Q[5]. Compound 2 contains R-Keggin-type [SiW12O40]4- heteropolyanions and Me10Q[5], whereas compound 3 is constructed from β-Keggin-type heteropolyoxosilicotungstate clusters [SiW12O40]4- and Me10Q[5]. All of the compounds feature train-like structural subunits based on central POM/alkali metal backbones and two side rows of parallel Me10Q[5] macrocycles.

Introduction Macrocycle cavitant of cucurbit [n=5-8,10]uril (Q[n]) and its derivatives have been extensively investigated as molecular hosts because of their hydrophobic cavity and two identical carbonyl fringed portals.1 Scientific research has demonstrated that the hydrophobic cavity of Q[n]s can bind a wide range of guests, especially some positively charged organic molecules.2 Moreover, the carbonyl-fringed portals of Q[n]s can coordinate to metal cations, preferably to alkali and alkaliearth metal ions.1 Q[n]s are, therefore, considered as a class of promising organic building blocks for the design and assembly of solid materials. Several Q[n]s precursors have already been exploited to construct different kinds of molecular devices, such as molecular containers,3 honeycombs,4 bowls,5 supramolecular bracelets,6 and gyroscanes.7 However, polyoxometalates (POMs) have drawn continuous research interest because of their distinctive nanosized structures and excellent acid/base or redox properties, leading to applications in many fields such as catalysis, biology, medicine, and so on.8 One recent progress in POM chemistry is the development of nanotechnology using the so-called bottom-up approach that favors the construction of new electronic/magnetic devices, machines, and materials by means of the incorporation of POMs with other functional organic subunits.9 Metalloporphyrins10 and some pyridyl based organic macromolecules11 have been previously investigated to construct hybrid materials with POMs. More recently, several fascinating examples of Q[n]-POM supramolecular materials were given birth. For instance, K€ ogerler and co-workers presented two Q[n]-POM hybrids using parent Q[6] and Q[8] to assemble with the [H2O@VIV18O42]12- polyoxoanion.12 Kasuga et al. reported a family of hybrid solids through the assembly of a decavanadate polyanion [V10O28(6 - n)-] and decamethylcucurbit[5]uril (Me10Q[5]).13 Contemporaneous research on supra*To whom correspondence should be addressed. Tel: þ86 591 83796710. Fax: þ86 591 83796710. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 02/19/2010

molecular {V10O28}-Me10Q[5] complexes were also carried out by our group, and various molecular ladder, sandwich, and chain structures were recorded.14 However, the current research on supramolecular Q[n]-POM chemistry is mainly limited in the assemblies of iso-polyoxovanadate subunit with various Q[n]s.12- There is no report on complexes constructed from Q[n]s with polymolybdates or polytungstates, belonging to two other extensively investigated POM subclasses, and more importantly, no heteropolyoxometalates have been successfully incorporated into hybrid Q[n]-POM solid materials so far. With an eye to push the boundaries of this burgeoning research area, we extended our attempt to the combination of polytungstates and Q[n]s. Three supramolecular compounds, [Na6(H2O)11(Me10Q[5])2] 3 [β-H2W12O40] 3 5H2O (1), [Na4(H2O)7(Me10Q[5])2] 3 [R-SiW12O40] 3 20H2O (2), and [Na4(H2O)7(Me10Q[5])2] 3 [β-SiW12O40] 3 13H2O (3) were successfully isolated. X-ray structural analyses show that compound 1 comprises β-Keggin-type iso-polyoxotungstate [H2W12O40]6- clusters and Me10Q[5]. Compounds 2 and 3 are constructed from R- and β-Keggin-type heteropolyoxosilicotungstate clusters [SiW12O40]4- and Me10Q[5], respectively, with alkali metal hydrates playing chargecompensating and linking roles. Guest water molecules were present in the crystal lattice of the three compounds. It is interesting that all of the compounds feature train-like structural subunits based on central POM/alkali metal backbones (body) and two side rows of parallel Me10Q[5] macrocycles (wheels). To the best of our knowledge, compounds 1-3 represent the first examples of the Q[n]polyoxotungstate family, and compounds 2 and 3 are unprecedented Q[n]-POM hybrids with hetero-POM clusters. Experimental Section General. All chemicals were commercially purchased and used without purification. Me10Q[5] was synthesized according to the literature.15 Elemental analyses (C, H, and N) were carried on an r 2010 American Chemical Society

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Table 1. Crystal Data and Structure Refinement for Compounds 1-3 Empirical formula formula weight temperature (K) wavelength (A˚) crystal system space group a (A˚) b (A˚) c (A˚) R (°) β (°) γ (°) volume (A˚3) Z goodness-of-fit on F2 final R indices [I > 2σ(I)] indices (all data)

C80H154N40O87W12Na4Si

C80H140N40O80W12Na4Si

5394.4 293(2) 0.71073 monoclinic C2/m 27.5763(2) 16.7925(9) 16.2791(1) 90 97.954(3) 90 7465.9(7) 2 1.115 R1a = 0.0749 wR2b = 0.2077 R1a = 0.0828 wR2b = 0.2143

5266.9 293(2) 0.71073 orthorhombic Pmmn 15.194(3) 32.016(6) 14.397(3) 90 90 90 7003(2) 2 1.127 R1a = 0.0541 wR2b = 0.1448 R1a = 0.0583 wR2b = 0.1479

)

R1 = Σ F0| - |Fc /Σ|F0|. b wR2 = {Σ[w(F02 - Fc2)2]/Σ[w(F02)2]}1/2 )

a

C80H134N40O76W12Na6 5214.8 293(2) 0.71073 orthorhombic Pmmn 15.097(3) 32.092(6) 14.403(3) 90 90 90 6978(2) 2 1.286 R1a = 0.0885 wR2b = 0.2161 R1a = 0.0931 wR2b = 0.2233

Elementar Vario EL III analyzer. Elemental analysis of Na was determined by a Jobin Yvon Ultima2 ICP atomic emission spectrometer. Infrared (IR) spectra were recorded with PerkinElmer Spectrum One (KBr pellets). X-ray powder diffractions (XRPD) were performed with a Rigaku DMAX 2500 diffractometer. The thermogravimetric analysis (TGA) was performed on a NETZSCH STA 449 F3 instrument in flowing N2 with a heating rate of 10 °C min-1. Synthesis. Compounds 1-3 were synthesized under typical hydrothermal conditions, and the successful isolation of the final products relied on a mutual dependence of the temperature and pH. [Na6(H2O)11(Me10Q[5])2] 3 [β-H2W12O40] 3 5H2O (1). Na2WO4 3 2H2O (0.75 g, 2.27 mmol) and Me10Q[5] (0.0773 g, 0.0773 mmol) were dissolved in 15 mL of distilled water. pH of the above mixture was adjusted to 4.5 by adding hydrochloric acid (5M). The mixture was stirred for another 15 min, then sealed in Teflon-lined stainlesssteel vessels under autogenous pressure at 140 °C for 3 days and cooled to room temperature within a day. Colorless crystals of compound 1 suitable for single crystal X-ray crystallography were collected by filtration. Yield, 57% (based on Me10Q[5]). Anal. Calcd for C80H134N40O76W12Na6 (M = 5214.8): C, 18.42; H, 2.56; N, 10.73; Na, 2.65; found, C, 18.24; H, 2.35; N, 10.21; Na, 2.75. [Na4(H2O)7(Me10Q[5])2] 3 [r-SiW12O40] 3 20H2O (2). Na2WO4 3 2H2O (0.75 g, 2.27 mmol), Na2SiO3 3 9H2O (0.050 g, 0.176 mmol), and Me10Q[5] (0.075 g, 0.0773 mmol) were dissolved in 15 mL of distilled water. Hydrochloric acid (5M) was added thereafter to adjust the pH higher than 8.0 and stirred for another 15 min (note: compound 2 can form in a wide pH range from 8.0 to 11.0). The mixture was then sealed in Teflon-lined stainless-steel vessels under autogenous pressure at 120 °C for 3 days and cooled to room temperature within a day. Colorless crystals of compound 2 were obtained in a yield of approximately 42% (based on Me10Q[5]). Anal. Calcd for C80H154N40O87W12Na4Si (M = 5394.4): C, 17.80; H, 2.88; N, 10.38; Na, 1.70; found, C, 17.84; H, 3.01; N, 10.36; Na, 1.76. [Na4(H2O)7(Me10Q[5])2] 3 [β-SiW12O40] 3 13H2O 3. Na2WO4 3 2H2O (0.75 g, 2.27 mmol), Na2SiO3 3 9H2O (0.050 g, 0.176 mmol), and Me10Q[5] (0.075 g, 0.0773 mmol) were dissolved in 15 mL of distilled water. Hydrochloric acid (5M) was added thereafter to adjust the pH to 7.0-8.0 and stirred for another 15 min. The mixture was then sealed in Teflon-lined stainless-steel vessels under autogenous pressure at 120 °C for 3 days and cooled to room temperature within a day. Colorless crystals of compound 3 were obtained in a yield of approximately 35% (based on Me10Q[5]). Anal. Calcd for C80H140N40O80W12Na4Si (M = 5266.9): C, 18.24; H, 2.66; N, 10.63; Na, 1.75; found, C, 18.10; H, 2.35; N, 10.24; Na, 1.73. Crystallography. X-ray Structure Determination. Single-crystal X-ray diffraction measurements of compounds 1-3 were carried out on a Rigaku diffractometer with a Mercury CCD area detector using graphite monochromated Mo KR radiation (λ=0.71073 A˚) at

Figure 1. Schematic view of Me10Q[5] and its crystal structure. 293(2) K . Empirical absorption corrections were applied to the data using the CrystalClear program.16 Structural solution and full matrix least-squares refinements based on F2 were performed with the SHELXS-97 and SHEXL-97 program packages, respectively.17 All of the non-hydrogen atoms were refined anisotropically. Crystal data and structure refinement parameters are given in Table 1. CCDC: 739343, 760538, and 739344 contain the supplementary crystallographic data for 1, 2, and 3. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Results and discussions Decamethylcucurbit[5]uril (Me10Q[5]) has a core structure similar to that of cucurbit[5]uril (Q[5]) with the outer face of the barrel equator decorated with methyl groups (Figure 1) and has a good solubility in water.15 Me10Q[5] can be considered as one of the smallest members of the cucurbit[n]uril family possessing comparatively narrow portals and a low cavity volume. Because of its low volume cavity, the supramolecular chemistry of Me10Q[5] is mainly targeted on the portals. Crystallographic investigations show that compunds 1-3 display similar train-like structural subunits based on central POM/alkali metal backbones and two side rows of parallel Me10Q[5] macrocycles. The main differences lies in the nature of Keggin clusters and the interconnection of alkali metals, Me10Q[5], and POM clusters. There are three independent sodium cations (Na1, Na2, and Na3), half of the Me10Q[5] molecule, quarter of the β-Keggin-type iso-polytungstate anion [H2W12O40]6-, and water molecules in the asymmetric unit of compound 1. The [H2W12O40]6- anion (Figure 2) consists of three {W4O17} units by sharing edges via W-O-W connections with a C3v symmetry to give a β-Keggin-type polyoxoanion shell. Unlike the usual structural pattern observed in hetero-Keggin type

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anions, the central part of the anions is not a heteroatom but two protons.18 The oxygen atoms on the polyoxoanion shell can be divided into three groups. Group I: each of the 12 terminal oxygen atoms bounds to one W with a W-O bond distance in the range of 1.63(2)-1.70(1) A˚. Group II: each of the 24 doubly bridging O atoms coordinates to two W atoms with W-O distances in the range of 1.86(2)-1.96(1) A˚. Group III: each of the four triply bridging O atoms links three W atoms with a W-O distance of 2.15(1)-2.29(1) A˚. As shown in Figure 3, one of the sodium ions (Na3) locates on the pseudocenter of the portal of Me10Q[5], coordinated with five carbonyl O atoms (O1, O2, O3, O20 , and O30 ) and two water molecules (O3W and O5W), with the Na-O distances being 2.27(2)-2.757(12) A˚. The other two sodium cations Na1 and Na2 locate on the rim of the other portal of the Me10Q[5]. Na1 is hepta-coordinated by four carbonyl oxygen atoms (O5 and O6 from the portal of cucurbituril, and O50 and O60 from the symmetric pair of Me10Q[5]), O16 from the terminal oxygen of the polyanion, and two water molecules (O1W and O2W) with the Na-O distances being 2.261(12)-2.909(13) A˚. Na2 is also hepta-coordinated by four carbonyl oxygen atoms (O4 and O5 from the portal of cucurbituril, and O40 and O50 from the symmetric pair of Me10Q[5]), three water molecules (O1W, O6W, and O9W), with the Na-O distances being 2.425(14)-2.70(6) A˚. Thus, the cations of Na1, Na2, Na10 , and Na20 connect the two cucurbiturils to form a cucurbituril dimer. The dimer serves as an ideal building block to assemble with [H2W12O40]6- anions. The β-Keggin [H2W12O40]6- anions join the building blocks via terminal coordination (Na1-O16) leading to the conformation of a train-like subunit (Figure 4), with Me10Q[5]

macrocycles as wheels and alkali metal/POM clusters as the body. Structural analysis of compound 2 shows that 2 possesses a supramolecular array of one-dimensional (1-D) train-like subunits constructed by Me10Q[5]s, POM moieties, and alkali metal ions (Figure S7, Supporting Information). The POM cluster in 2 is the well-known R-Keggin-type heteropolyoxosilicotungstate (Figure 2), which is built by four groups of trimetallic {WO6}3 units in a Td symmetry. Each {WO6} octahedron in one trimetallic unit shares an edge with a neighboring one, and the {WO6}3 units link together via corner sharing {WO6} octahedra to form a cluster cage with a disordered {SiO8} cube located in the center of the host cage (the eight oxygen atoms are half-occupied). The W-O bond lengths and W-O-W angles are in the normal range observed in Keggin-type heteropolytungstates.19 Two Me10Q[5] units form a dimer connected by four sodium atoms, all half-occupied, through the carbonyl oxygen atoms at the portal. The dimer units are further linked into train-like chains by R-Keggin POM clusters. Single crystal X-ray analysis shows that in the structure of compound 3, two Me10Q[5]s form a dimer building unit which is very similar to that of compounds 1 and 2, but only two sodium cations (Na2 and Na20 ) that serve as connectors (Figure 3c). The POM subunit in 3 is a β-Keggin-type heteropolyoxosilicotungstate cluster exhibiting a similar host shell with a C3v symmetry as in compound 1, in which a disordered {SiO8} cubic moiety was encapsulated with Si-O distances being 1.574(14)-1.670(14) A˚ (Figure 2c). The β-Keggin-type polyoxosilicotungstate cluster has been proved a less stable isomer of the R-Keggin-type analogue and has been rarely reported so far.19c The W-O bond lengths and W-O-W

Figure 2. Polyhedral of β-[H2W12O40]6- (left), R-[SiW12O40]4(middle), and β-[SiW12O40]4- (right).

Figure 4. Representative view of the train-like subunits in compound 1.

Figure 3. View of the linkage between two Me10Q[5] units in compounds 1-3. Only atoms in an asymmetric unit are labeled and shown in color. Color codes: C, dark gray; N, cyan; O, red; Na, purple.

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from 20 to 250 °C are assigned to the removal of the coordinated and uncoordinated water molecules, which are slightly higher than the calculated values because of the physical absorption of surface water. The main weight losses from 400 to 800 °C are attributable to the decomposition of Me10Q[5], which are identical to the calculated values. TG analyses show that the stabilities of the compounds are comparable to pure Q[n]s (decompose around 350 °C)1a as indicated by the platforms appearing between 250 and 400 °C. Conclusions

Figure 5. TG curves of compounds 1-3.

angles are in the normal range observed in Keggin-type heteropolytungstates.20 The β-Keggin anions [SiW12O40]4and Me10Q[5] dimers aggregate into the train-like chain structure by means of the coordination of sodium cations (Figure S7, Supporting Information). The packing patterns of Me10Q[5] and POM clusters in the unit cell of compounds 1 and 3 are similar, and the cell parameters are almost equivalent, although the numbers of lattice water molecules and counterions are different. In the crystal structures of compounds 1-3, the basic trainlike building units are extended into three-dimensional (3-D) supramolecular architectures through the extensive hydrogen interactions between disassociated water molecules, surface oxygen atoms of POMs, and portal carbonyl oxygen atoms of Me10Q[5]s. Typical hydrogen bonds are in the range of 2.461-2.958 A˚ (Ow 3 3 3 H 3 3 3 Ow), 2.455-3.009 A˚ (Ow 3 3 3 H 3 3 3 Ocarbonyl), and 2.382-3.013 A˚ (Ow 3 3 3 H 3 3 3 OPOM) (Table S1, Supporting Information). The shape of the train-like subunit in compound 2 is a little different from the subunits in compounds 1 and 3 because of a different connection mode that POM clusters link to the sodium atoms in between the Me10Q[5] dimers. The R-Keggin-type [SiW12O40]4- anions in 2 connect to the μ-3 O atoms, which also link to two other sodium atoms, whereas the β-Keggin-type [H2W12O40]6- and [SiW12O40]4- anions in compounds 1 and 3 bound to the μ-2 O atoms of the Me10Q[5] dimer moieties (Figure S5, Supporting Information). Thus, in compound 2 the POM clusters locate almost to the same mean plane as the Me10Q[5] dimers (Figure S6, middle (Supporting Information)). However, in compounds 1 and 3, the POM moieties sit out from the mean plane of the Me10Q[5] dimers arranging in a triangular pattern (Figure S6, top and bottom (Supporting Information)). Thanks to the nanosized molecular components of compounds 1-3, the train-like subunits can be considered as molecular nanowires, which may have potentially interesting physical properties and applications in the field of nanomaterials. The phase purity of compounds 1-3 was confirmed by XRPD patterns (Figure S2-S4, Supporting Information). Peak positions of the experimental and simulated spectra are in good agreement with each other indicating the pure phase of the bulky products. The thermolgravitmetric (TG) curves of the as-synthesized compounds 1-3 all exhibit similar two steps of weight losses (Figure 5). The weight losses

In conclusion, we report a reasonable way to build inorganic-organic hybrid solid materials through the aggregation of wheel-like cucurbituril subunits and POMs. The as-synthesized compounds display new structures with train-like building units, derived from the versatile coordination modes between Me10Q[5]s, POMs, and alkali metals. The interesting train-like subunits in compounds 1-3 exhibit nanosized structural dimensions, awakening reminiscences of inorganic-organic hybrid molecular nanowires originated from nanosized components (Me10Q[5] and POMs). It may open up possibilities in the syntheses of hybrid nanomaterials. Acknowledgment. We thank the 973 Program (2006CB932903, 2007CB815303), NSFC (20731005, 20821061, 20873151), Fujian Key Laboratory of Nanomaterials (2006L2005), and Key projects from CAS for financial support. Supporting Information Available: Summary of H bonds in compounds 1-3, IR spectra of compounds 1-3, experimental and simulated XRPD spectra of compounds 1-3, a view of the connection between POM clusters and sodium-oxo moieties in compounds 1-3, a front view of the train-like subunits in compounds 1-3, and a representative view of the train-like subunits in compounds 2 and 3, and crystallographic information files (CIF format) of crystals 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.

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