One-dimensional ion transport in self-organized columnar ionic liquids

New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials. The ionic liquids consist of two incompatible parts: an imidazolium-based ionic part as an ion-conducting part and tris(alkyloxy)phenyl parts as insulating parts. Two compounds having octyl and dodecyl chains have been synthesized. Self-assembly of these materials leads to the formation of thermotropic hexagonal columnar liquid crystalline states at room temperature. Anisotropic one-dimensional ionic conductivities have been successfully measured by the cells having comb-shaped gold electrodes. The self-organized columns have been aligned macroscopically in two directions by shearing perpendicular and parallel to the electrodes. The ionic conductivities parallel to the column axis are higher than those perpendicular to the axis. The incorporation of lithium salts in these columnar materials leads to the enhancement of the ionic conductivities and their anisotropy. These materials would be useful for anisotropic transportation of ions at the nanometer level.     

Synthesis and structures of niobium(V) complexes stabilized by linear-linked aryloxide trimers

The preparation and characterization of a series of niobium(V) complexes that incorporate the linear-linked aryloxide trimers 2,6-bis(4,6-dimethylsalicyl)-4-tert-butylphenol [H3(Me-L)] and 2,6-bis(4-methyl-6-tert-butylsalicyl)-4-tert-butylphenol [H3(tBu-L)] are described. The chloride complex [Nb(Me-L)Cl2]2 (1) was prepared in high yield by reaction of NbCl5 with H3(Me-L) in toluene. In contrast, the analogous reaction with H3(tBu-L) gave a mixture of [Nb(tBu-L)Cl2]2 (2) and [Nb(de-tBu-L)Cl2]2 (3a). During the formation of 3a, one of tert-butyl groups at the ortho position in the tBu-L ligand was lost. When the NbCl5/H3(tBu-L) reaction was carried out in acetonitrile, Nb[H(tBu-L)]Cl3(NCMe) (4) was obtained. Heating a solution of 4 in toluene generated 2 and 3a. The isolated complex 4 underwent ligand redistribution in acetonitrile to produce Nb[H(tBu-L)]2Cl(NCMe) (5). Treatment of NbCl5 with Li3(tBu-L) in toluene afforded 2. The chloride ligands in 1 and 2 smoothly reacted with 4 equiv of MeMgI and LiStBu, resulting in [Nb(R-L)Me2]2 [R = Me (6), tBu (7)] and Nb(Me-L)(StBu)2 (8), respectively. A number of the above complexes have been characterized by X-ray crystallography. In the structures of 1, 2, and 6, the R-L ligand is bound to the metal center with a U-coordination mode, while an alternative S-conformation is adopted for 3a and 8. Complexes 4 and 5 contain a bidentate H(tBu-L) diphenoxide-monophenol ligand.     

Seven-membered vibsane-type diterpenes with a 5,10-cis relationship from Viburnum awabuki

New five seven-membered vibsane-type diterpenes named 5-epi-vibsanin C, 5-epi-vibsanin H, 5-epi-vibsanin K, 18-O-methyl-5-epi-vibsanin K and 5-epi-vibsanin E have been isolated from the leaves of Viburnum awabuki (Caplifoliaceae). Their structures have been elucidated by analyses of spectroscopic data and comparison of their spectral data with those of the previously known seven-membered vibsane-type diterpenes. The occurrence of these seven-membered vibsane-type diterpenes with a cis relationship on the C-5 and C-10 positions in nature have been predicted by conformational analysis of vibsanin B, an eleven-membered vibsane-type diterpene. Vibsanin C, 5-epi-vibsanin C and 5-epi-vibsanin H exhibited moderate cytotoxic activities on KB cells.     

A modified method using static head-space gas chromatography for determining the stability constants of 1-alkanol/alpha-cyclodextrin complexation

A modification of the conventional static head-space gas chromatography method (SHSGC method) to determine stability constants for 1-alkanol/alpha-CD inclusion complexes was investigated. The 1 : 1 stability constants determined by this modified SHSGC method are in reasonable agreement with the corresponding values reported previously. The modified SHSGC method precludes the necessity of the calibration curve by the use of Henry's law constant of guest. Consequently, the modified SHSGC method is more advantageous than the conventional SHSGC method because the experimental time required for determination of the stability constant is markedly reduced.     

Complexes of tantalum with triaryloxides: Ligand and solvent effects on formation of hydride derivatives

A family of tantalum compounds supported by the triaryloxide [R-L]³⁻ ligands are reported [H₃(R-L) = 2,6-bis(4-methyl-6-R-salicyl)-4-tert-butylphenol, where R = Me or ^tBu]. The reaction of H₃[Me-L] with TaCl₅ in toluene gave [(Me-L)TaCl₂]₂ (1). The [^tBu-L] analogue [(^tBu-L)TaCl₂]₂ (2) was synthesized via treatment of TaCl₅ with Li₃[^tBu-L]. A THF solution of LiBHEt₃ was added to 1 in toluene to provide [(Me-L)TaCl(THF)]₂ (3), while treatment of 2 with 2 equiv of LiBHEt₃ or potassium in toluene followed by recrystallization from DME resulted in formation of [M(DME)₃][{(^tBu-L)TaCl}₂(μ-Cl)] [M = Li (4a), K (4b)]. When the amount of MBHEt₃ (M = Li, Na, K) was increased to 5 equiv, the analogous reactions in toluene afforded [{(bit-^tBu-L)Ta}₂(μ-H)₃M] [M = Li(THF)₂ (5a), Na(DME)₂ (5b), K(DME)₂ (5c)]. During the course of the reaction, the methylene CH activation of the ligand took place. Dissolution of 5a in DME produced [{(bit-^tBu-L)Ta}₂(μ-H)₃Li(DME)₂] (6), indicating that the coordinated THF molecules are labile. When the 2/LiBHEt₃ reaction was carried out in THF, the ring opening of THF occurred to yield [(^tBu-L)Ta(OBuⁿ)₂]₂ (7) along with a trace amount of [Li(THF)₄][{(^tBu-L)TaCl}₂(μ-OBuⁿ)] (8). Treatment of 2 with potassium hydride in DME yielded [{(^tBu-L)TaCl₂K(DME)₂}₂(μ-OCH₂CH₂O)] (9), in which the ethane-1,2-diolate ligand arose from partial C-O bond rupture of DME. The X-ray crystal structures of 2, 3, 4, 5a, 6, 7, and 9 are described.     

Kinetically controlled Ag+-coordinated chiral supramolecular polymerization accompanying a helical inversion

We report kinetically controlled chiral supramolecular polymerization based on ligand–metal complex with a 3 : 2 (L : Ag⁺) stoichiometry accompanying a helical inversion in water. A new family of bipyridine-based ligands (d-L1, l-L1, d-L2, and d-L3) possessing hydrazine and d- or l-alanine moieties at the alkyl chain groups has been designed and synthesized. Interestingly, upon addition of AgNO₃ (0.5–1.3 equiv.) to the d-L1 solution, it generated the aggregate I composed of the d-L1AgNO₃ complex (d-L1 : Ag⁺ = 1 : 1) as the kinetic product with a spherical structure. Then, aggregate I (nanoparticle) was transformed into the aggregate II (supramolecular polymer) based on the (d-L1)₃Ag₂(NO₃)₂ complex as the thermodynamic product with a fiber structure, which led to the helical inversion from the left-handed (M-type) to the right-handed (P-type) helicity accompanying CD amplification. In contrast, the spherical aggregate I (nanoparticle) composed of the d-L1AgNO₃ complex with the left-handed (M-type) helicity formed in the presence of 2.0 equiv. of AgNO₃ and was not additionally changed, which indicated that it was the thermodynamic product. The chiral supramolecular polymer based on (d-L1)₃Ag₂(NO₃)₂ was produced via a nucleation–elongation mechanism with a cooperative pathway. In thermodynamic study, the standard ΔG° and ΔH_e values for the aggregates I and II were calculated using the van''t Hoff plot. The enhanced ΔG° value of the aggregate II compared to that of the formation of aggregate I confirms that aggregate II was thermodynamically more stable. In the kinetic study, the influence of concentration of AgNO₃ confirmed the initial formation of the aggregate I (nanoparticle), which then evolved to the aggregate II (supramolecular polymer). Thus, the concentration of the (d-L1)₃Ag₂(NO₃)₂ complex in the initial state plays a critical role in generating aggregate II (supramolecular polymer). In particular, NO₃⁻ acts as a critical linker and accelerator in the transformation from the aggregate I to the aggregate II. This is the first example of a system for a kinetically controlled chiral supramolecular polymer that is formed via multiple steps with coordination structural change.

The nanoparticles were transformed into the supramolecular polymer as the thermodynamic product, involving a helical inversion from left-handed to right-handed helicity.     

Convergent synthesis of the ABCDE ring fragment of ciguatoxins

Synthesis of the ABCDE ring fragment of ciguatoxins has been achieved in a highly stereocontrolled and convergent manner via the B-alkyl Suzuki-Miyaura coupling-based approach.     

Electrostatic interaction between ion-penetrable membranes in a salt-free medium

A set of coupled equations is given which determines the distributions of the electric potential and counterions in a system of two interacting identical ion-penetrable membranes of thickness d at separation h immersed in a salt-free medium containing only counterions. The solution to these coupled equations also gives the electrostatic repulsive force between the membranes. It is shown that the interaction force remains finite at h-->0, unlike the case of the interaction between two planar charged surfaces (d-->0), and that the interaction force becomes independent of the membrane fixed charge and membrane thickness d at very large h. Finally, an approximate single transcendental equation giving the solution to the coupled equations is derived.     

Ketene formation in benzdiyne chemistry: ring cleavage versus Wolff rearrangement

In the chemistry toward generating benzdiyne from five benzenetetracarboxylic dianhydride derivatives, ketene formation was exclusively observed in the photolysis of difluorobenzenetetracarboxylic dianhydride in a nitrogen matrix at 13 K. Two ketenes were formed concomitantly with difluorobenzdiyne. These ketenes were identified on the basis of good agreement between the observed and calculated (B3LYP/6-31G level) IR spectra. Neither ketene contained the five-membered-ring moiety as cyclopentadienylideneketene, which is formed by Wolff rearrangement in the benzyne chemistry. The first generated ketene was assigned to a ketene with a cyclopropene moiety, and the second, to a ketene with a butadiyne moiety. The first generated ketene was a major product in the photolysis and was formed by cleavage of the bond connecting the ketene group and the C-F carbon and not the bond connecting the ketene group and the carbene moiety. Thus the structures of these ketenes indicated that a unprecedented ring cleavage, rather than Wolff rearrangement, is the dominant process in the benzdiyne chemistry.     

Efficient Separation of Hydrophobic Molecules by Molecularly Imprinted Cyclodextrin Polymers

Cyclodextrins were cross-linked with toluene 2,4-diisocyanate in dimethyl sufoxide in the presence of hydrophobic biomolecules as templates, and the imprinted polymers were applied to the stationary phases of high performance liquid chromatography. Molecular imprinting efficiently promoted the binding-affinity and substrate-selectivity towards the template molecule, compared with the control polymers prepared in their absence. When cholesterol (template molecule) was complexed with cyclodextrins prior to the polymerization, for example, the imprinted polymer retained cholesterol more strongly than other steroids. Upon the polymerization without a template molecule, the binding towards steroids was much weaker. Besides steroids, imprinting was effective for various hydrophobic and rigid template molecules. Since binding of the guest molecule was based on inclusion complex formation with cyclodextrins, separation could be achieved in the solvents containing water. These polymeric receptors are also applicable to selective recognition of biologically important molecules or removal of toxic molecules from aqueous media. Thus, imprinting of cyclodextrins is useful for the preparation of synthetic tailor-made receptors for various kinds of hydrophobic guest molecules.