Sayaka Uchida

Cucurbit[n]uril−Polyoxoanion Hybrids

The first organic-inorganic hybrid complexes between CB[n] and polyoxometalates not only display a surprisingly high structural complementarity, the right pairing also allows their chemical and physical properties to be coupled, as illustrated by two examples.

Micelles and Vesicles Formed by Polyoxometalate–Block Copolymer Composites

Polyoxometalates (POMs) are anionic metal oxide clusters with several to a few tens of negative charges, and their structural and electronic versatility has resulted in various applications in the fields of catalysis, biomedicine, and materials science. However, these hydrophilic clusters are basically incompatible with hydrophobic organic materials and have high lattice energies associated with crystallization. Therefore, further fabrication of POM-based materials and devices requires manipulating the clusters on the nanoscale through solution-based self-assembly. The surface properties of POMs can be modified by replacement of the countercations with cationic surfactants to form surfactant-encapsulated clusters (SECs). The resultant SECs are soluble in organic media and facilitate fabrication of POM-based thin films such as Langmuir monolayers, Langmuir–Blodgett films, and solvent-cast films. On the other hand, ionic block copolymers are macromolecular analogues of conventional ionic surfactants. They can self-assemble into regular and reverse micellelike nanosized aggregates in aqueous media and nonpolar organic solvents, respectively. The morphologies of the aggregates primarily depend on the incompatibility between blocks, block compositions, and solvents. Poly(styrene-b-4-vinyl-N-methylpyridinium iodide), Sn-bVm, with long S and short V blocks can form multiple morphologies of aggregates (i.e., sphere, cylinder, and bilayer) on dispersion in water. However, only reverse spherical micelles have been reported for this block ionomer family in toluene. Combination of Sn-b-Vm with oppositely charged block ionomers leads to formation of asymmetric vesicles. Here we report the preparation of POM/block copolymer composites and their self-assembly into micelles and vesicles in organic solvents. The micelles reach the superstrong segregation (SSS) regime, where the ionic core radii correspond to the length of fully stretched V blocks, the interface is totally occupied by S/V junction points, and no more space is available to incorporate chains. The POM/Sn-b-Vm (SVP) composites (Figure 1) were prepared by electrostatic incorporation of POMs into solid Sn-b-Vm matrices as follows: Na3[a-PW12O40] was dissolved in water and the pH value modified to 0.7–0.8 with aqueous

Synthesis of a dialuminum-substituted silicotungstate and the diastereoselective cyclization of citronellal derivatives.

A novel dialuminum-substituted silicotungstate TBA(3)H[gamma-SiW(10)O(36){Al(OH(2))}(2)(mu-OH)(2)] x 4 H(2)O (1, TBA = tetra-n-butylammonium) was synthesized by the reaction of the potassium salt of [gamma-SiW(10)O(36)](8-) (SiW10) with 2 equiv of Al(NO(3))(3) in an acidic aqueous medium. It was confirmed by the X-ray crystallographic analysis that compound 1 was a monomer of the gamma-Keggin dialuminum-substituted silicotungstate with the {Al(2)(mu-OH)(2)} diamond core. The cluster framework of 1 maintained the gamma-Keggin structure in the solution states. The reaction of 1 with pyridine yielded TBA(3)[(C(5)H(5)N)H][gamma-SiW(10)O(36){Al(C(5)H(5)N)}(2)(mu-OH)(2)] x 2 H(2)O (2), and the molecular structure was successfully determined by the X-ray crystallographic analysis. In compound 2, two of three pyridine molecules coordinated to the axial positions of aluminum centers and one of them existed as a pyridinium cation, showing that compound 1 has two Lewis acid sites and one Brønsted acid site. Compound 1 showed high catalytic activity for the intramolecular cyclization of citronellal derivatives such as (+)-citronellal (3) and 3-methylcitronellal (4) without formation of byproduct resulting from etherification and dehydration. For the 1-catalyzed cyclization of 3, the diastereoselectivity toward (-)-isopulegol (3a) reached ca. 90% and the value was the highest level among those with reported systems so far. The reaction rate for the 1-catalyzed cyclization of 3 decreased by the addition of pyridine, and the cyclization hardly proceeded in the presence of 2 equiv of pyridine with respect to 1. On the other hand, the reaction rate and diastereoselectivity to 3a in the presence of 2,6-lutidine were almost the same as those in the absence. Therefore, the present cyclization is mainly promoted by the Lewis acid sites (aluminum centers) in 1. DFT calculations showed that the formation of the transition state to produce 3a is sterically and electronically more favorable than the other three transition states for the present 1-catalyzed cyclization of 3.

Highly selective sorption of small unsaturated hydrocarbons by nonporous flexible framework with silver ion.

Ag2[Cr3O(OOCC2H5)6(H2O)3]2[alpha-SiW12O40] [1] is a nonporous flexible ionic crystal composed of 2D-layers of polyoxometalates ([alpha-SiW12O40](4-)) and macrocations ([Cr3O(OOCC2H5)6(H2O)3](+)) stacking along the b-axis. The silver ions are located in the vicinity of the oxygen atoms of the polyoxometalates. The sorption amounts of small unsaturated hydrocarbons such as ethylene, propylene, n-butene, acetylene, and methyl acetylene into 1 are comparable to or larger than 1.0 mol mol(-1) and large hystereses are observed, while those of paraffins and larger unsaturated hydrocarbons are smaller than the adsorption on the external surface (<0.2 mol mol(-1)). Fine crystals of 1 exhibit ethylene/ethane and propylene/propane sorption ratios over 100 at 298 K and 100 kPa, and the values are larger by 1 order of magnitude among those reported. The results of sorption kinetics, in situ IR spectroscopy, single crystal X-ray crystallography, and in situ powder XRD studies show that small unsaturated hydrocarbons penetrate into the solid bulk of 1 through the pi-complexation with Ag(+). The sorption property of 1 is successfully applied to the collection of ethylene from the gas mixture of ethane and ethylene.

A Tin–Tungsten Mixed Oxide as an Efficient Heterogeneous Catalyst for CC Bond-Forming Reactions

The tin-tungsten mixed oxide prepared by the calcination of the tin-tungsten hydroxide precursor with a Sn/W molar ratio of 2 at 800 degrees C (SnW2-800) acts as an effective and reusable solid catalyst for C-C bond-forming reactions, such as the cyclization of citronellal, the Diels-Alder reaction, and the cyanosilylation of carbonyl compounds with trimethylsilyl cyanide (TMSCN). Various kinds of structurally diverse aliphatic, aromatic, and unsaturated, heteroatom-containing substrates could be converted into the desired products in high to excellent yields. The observed catalyses for these reactions were truly heterogeneous and the recovered catalyst could be reused several times without an appreciable loss of its high catalytic performance. The Brønsted acid sites generated on the aggregated polytungstate species on SnW2-800 likely play an important role in the C-C bond-forming reactions.

A Flexible Nonporous Heterogeneous Catalyst for Size‐Selective Oxidation through a Bottom‐Up Approach

The bottom-up approach has the potential to create novel devices with a wide range of applications such as in electronics, medicine, and energy, as the arrangement of molecular building blocks into nanostructures can be controlled. 2] It is still a great challenge to fabricate not only devices but also heterogeneous catalysts with intended structures and functions by a bottom-up approach, while biominerals such as shells and bones have been already formed by the bottom-up approach through the self-assembly of inorganic building blocks with organic molecules in water. The control of the self-organization of nanobuilding blocks with well-defined sizes, shapes, and physical and chemical properties would lead to progress in science and technology. Various catalytically active sites, such as metal nodes, framework nodes, and molecular species, can be introduced into metal–organic frameworks (MOFs) through self-assembly. Efficient sizeand enantioselective catalysis by crystalline and porous MOFs has been reported for reduction, C C bond formation, and acid–base reactions, and hydrolytic and oxidative stabilities are critical for the development of MOF-based oxidation systems that are efficient, chemoand size-selective, and recyclable, and use the green oxidant H2O2. [5–7] Therefore, the development of efficient, easily recoverable, and recyclable heterogeneous oxidation catalysts with H2O2 by a bottom-up approach has received particular research interest. Polyoxometalates (POMs) are discrete early transitionmetal oxide cluster anions with applications in broad fields, such as catalysis, materials, and medicine, because their structures and chemical properties can be finely tuned by choose of the constituent elements. Various POMs such as peroxometalates, lacunary POMs, and transition-metal-substituted POMs have been developed for H2O2or O2-based green oxidations. Therefore, POMs are suitable nanobuilding blocks to construct heterogeneous oxidation catalysts. Recently, the development of heterogeneous oxidation catalysts based on POMs and the related compounds has been attempted according to the following strategies: “solidification” of POMs (formation of insoluble solid ionic materials with appropriate countercations) and “immobilization” of POMs through adsorption, covalent linkage, and ion exchange. In most cases, however, the catalytic activities and selectivities of the parent homogeneous POMs are somewhat or much decreased by the heterogenization, and there are only a few successful examples. We are interested in a bottom-up approach to the design and synthesis of artificial heterogeneous catalysts with POMs and herein report that the nonporous tetra-n-butylammonium salt of [g-SiW10O34(H2O)2] 4 ([(n-C4H9)4N]4[g-SiW10O34(H2O)2]·H2O, 1·H2O) synthesized through a bottom-up approach sorbs ethyl acetate (EtOAc), which is highly mobile in the solid bulk of the compound, probably contributing to the easy co-sorption of the olefins and H2O2. The compound heterogeneously catalyzes size-selective oxidation of various organic substances including olefins, sulfides, and silanes with aqueous H2O2 in EtOAc. The compound can easily be separated by filtration and reused several times with retention of its high catalytic activity. The catalysis is truly heterogeneous in nature because the filtrate after removal of the solid catalyst is completely inactive. Notably, sizeselective oxidation catalysis is observed: small olefins are much more preferentially epoxidized than large olefins. To the best of our knowledge, this study provides the first example for the heterogeneously catalyzed size-selective liquid-phase oxidation with H2O2 by a POM-based catalyst. Compound 1·H2O was synthesized by a bottom-up approach as described below. The silicodecatungstate [gSiW10O34(H2O)2] 4 was synthesized in situ by the addition of concentrated HNO3 to an aqueous solution of [g-SiW10O36] 8 . Then, tetra-n-butylammonium bromide [(n-C4H9)4N]·Br was added to the solution, and white powder of 1·H2O was formed. The use of other cations, such as tetramethylammonium [(CH3)4N] , formed single crystals. The powder Xray diffraction (XRD) pattern, crystal structure, and spacefilling model of 1·H2O are shown in Figure 1a–c. The [*] Prof. Dr. N. Mizuno, Dr. S. Uchida, Dr. K. Kamata, R. Ishimoto, S. Nojima Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)

Zeotype Organic–Inorganic Ionic Crystals: Facile Cation Exchange and Controllable Sorption Properties

Storage and selective sorption of molecules by porous materials such as zeolites and metal–organic frameworks (MOFs) have been areas of intensive research because of their importance in industrial and environmental fields. Recent studies have focused on rational design of porous materials by careful choice of precursors and starting materials. For example, the pore sizes and volumes of Zn dicarboxylates are finely controlled by the lengths of the dicarboxylate ligands. On the other hand, ion exchange is a facile method to control the pore structures and sorption properties of porous materials. Pore sizes and guest affinities of zeolites can be controlled by cation exchange. For example, K resides in the pores of K-LTA zeolite, and the effective pore diameter is 3 . The effective pore size is increased to 4 by exchange of K with the smaller Na. The amounts of methane and water adsorbed by alkaline earth metal ion-exchanged and alkali metal ion-exchanged FAU zeolites increase with increasing ionic potential Z/r (Z and r are the charge and radius of the ion, respectively) of the countercations. 4] The control of pore structures and sorption properties of MOFs by ion exchange is still rare compared with zeolites. For example, the pore volumes of A indium tetracarboxylates, and the H2 sorption properties of M II

Size‐Selective Sorption of Small Organic Molecules in One‐Dimensional Channels of an Ionic Crystalline Organic–Inorganic Hybrid Compound Stabilized by π–π Interactions

The design and syntheses of porous materials such as zeolites and metal–organic frameworks (MOFs) are areas of intense research because of their unique properties in gas storage, separation, and heterogeneous catalysis. Crystalline microporous zeolites show shape-selective adsorption properties because the sizes of the channel apertures formed by the covalently bonded [TO4] (T= Si, Al, P, Ti, etc.) and/or [MO6] (M = V, Mn, Mo, etc.) units can be controlled. In contrast, the pore sizes of MOFs, which are constructed from coordinatively bonded metal ions and organic ligands, can be controlled by the lengths and functional groups of organic ligands. In particular, the use of aromatic units induces p–p interactions and enables the rational control of the complexation of the building units. In addition, the presence of the aromatic moieties means that the resulting framework would show unique structural and guest-sorption properties. For example, the guest sorption in a Cu dihydroxybenzoate/4,4’-bipyridine MOF causes the gliding of p–p-stacked building units, which leads to the change in the framework structure, and a Co benzenetricarboxylate MOF separates the aromatic units from the aliphatic hydrocarbons. Trinuclear metal carboxylates have often been used as building units for the construction of MOFs, which mostly contain dicarboxylates as bridging ligands to connect the trinuclear metal units, and do not show p–p interactions among the building units. 4a–c] The use of aromatic units as terminal ligands of trinuclear metal carboxylates would induce p–p interactions, which lead to the unique structural and guest sorption properties. Based on these considerations, we have used pyridine as a terminal ligand of the trinuclear metal carboxylate macrocation [Cr3O(OOCH)6(pyridine)3] +

Ionic Crystals [M3O(OOCC6H5)6(H2O)3]4[α-SiW12O40] (M = Cr, Fe) as Heterogeneous Catalysts for Pinacol Rearrangement

Complexation of trinuclear oxo-centered carboxylates with a silicododecatungstate resulted in the formation of ionic crystals of [M(3)O(OOCC(6)H(5))(6)(H(2)O)(3)](4)[α-SiW(12)O(40)]·nH(2)O·mCH(3)COCH(3) [M = Cr (Ia), Fe (IIa)]. Treatments of Ia and IIa at 373 K in vacuo formed guest-free phases Ib and IIb, respectively. Compounds Ib and IIb heterogeneously catalyzed the pinacol rearrangement to pinacolone with high conversion at 373 K, and the catalysis is suggested to proceed size selectively in the solid bulk.

Control of Structures and Sorption Properties of Ionic Crystals of A2[Cr3O(OOCC2H5)6(H2O)3]2[α-SiW12O40] (A = Na, K, Rb, NH4, Cs, TMA)

The complexation of Keggin-type polyoxometalate [alpha-SiW 12O 40] (4-), macrocation [Cr 3O(OOCC 2H 5) 6(H 2O) 3] (+), and monovalent cation A (+) forms ionic crystals of A 2[Cr 3O(OOCC 2H 5) 6(H 2O) 3] 2[alpha-SiW 12O 40]. nH 2O [A = Na ( 1a), K ( 2a), Rb ( 3a), NH 4 ( 4a), Cs ( 5a), and tetramethylammonium (TMA) ( 6a)]. Single crystal (1a- 4a and 6a) and powder (5a) X-ray analyses have shown that the ionic crystals possess 2D layers of polyoxometalates and macrocations. Compounds 2a- 5a had almost the same structure, while the layers in 1a and 6a stack in different ways. The structures and sorption properties of 2b- 5b are investigated in more detail. The interlayer distances of guest free phases 2b- 5b increase with the increase in the ionic radii of the monovalent cations, which reside between the layers. Compounds 2b- 5b possess hydrophobic and hydrophilic channels, which exist between the layers and through the layers, respectively. The volumes of the hydrophobic channels increase in the order of 2b < 3b approximately 4b < 5b, and those of the hydrophilic channels increase in the order of 2b < or = 3b < or = 4b < 5b. Single-crystal X-ray structure analyses of 2a- 4a have shown that the water of crystallization resides in the hydrophilic channel. It is probable that the water of crystallization in 5a resides in the hydrophilic channel in the same manner as those in 2a- 4a since 2a- 5a have almost the same structure. The water vapor sorption profiles of 2b- 5b are approximately reproduced by a linear driving force model. Therefore, water molecules sorbed in 2b- 5b probably reside in the hydrophilic channel. The n-propanol sorption profiles are reproduced by the summation of the linear driving force model, showing that two independent barriers exist in the n-propanol sorption. The in situ IR spectra of n-propanol sorbed showed the presence of two n-propanol species. These data show that n-propanol is sorbed into both hydrophilic and hydrophobic channels. Compound 5b sorbs halocarbons in the hydrophobic channel, while 2b- 4b exclude them.