Koichi Ishibashi

Azacalixarene: A New Class in the Calixarene Family

Calixarenes, together with crown ethers and cyclodextrins, play an important role in supramolecular chemistry. A variety of calixarene analogues involving heteroatoms as the bridging units have been reported because the substitution of the carbon bridges with heteroatoms can impart novel properties and functions to molecules. This is typified by, for example, thiacalixarenes. In recent years, nitrogen-bridged calixarene analogues have emerged as a new calixarene family. While the diversity is still limited as compared with carbon- and sulfur-bridged calixarenes, intriguing structure-property relationships based on the introduction of nitrogen atoms as the bridging units have been reported. This review summarizes the recent reports on the preparations, conformations, and inclusion properties of nitrogen-bridged calixarene analogues with a [1 n ]metacyclophane skeleton.

Azacalix[6]arene Hexamethyl Ether: Synthesis, Structure, and Selective Uptake of Carbon Dioxide in the Solid State

To investigate dynamic solid-state complexation hitherto unexplored in nitrogen-bridged calixarene analogues, azacalix[6]arene hexamethyl ether has been prepared in three steps by applying a 5+1 fragment-coupling approach by using a Buchwald- Hartwig aryl amination reaction with the aid of our previously devised temporal N-silylation protocol. X-ray crystallographic analysis and NMR spectroscopic measurements have revealed that the azacalix[6]arene is well endowed with hydrogen-bonding ability, by which both the molecular and crystal structures are controlled. The azacalix[6]arene is conformationally flexible in solution on the NMR time scale, whereas it adopts a definite 1,2,3-alternate conformation with S2 symmetry in the solid state as a result of intramolecular bifurcated hydrogen-bonding interactions. In the crystal, molecules of the azacalix[6]arene are mutually interacted by intermolecular hydrogen bonds to establish one-dimensional hexane-filled nanochannel crystal architecture. Although the single crystal was broken after desolvation, the resultant polycrystalline powder material was capable of selectively adsorbing CO2 among the four main gaseous components of the atmosphere. In contrast, carbocyclic p-tert-butylcalix[6]arene hexamethyl ether, the crystal structure of which was also elucidated for the first time in the present study, gave rise to almost no uptake of CO2. Additional solid-gas adsorption experiments for another three gases, such as N2, O2, and Ar, suggested that quadrupole/induced-dipole interactions and/or hydrogen-bonding interactions played an important role in permitting the observed selective uptake of CO2 by this new azacalix[6]arene in the solid state.

Spontaneous and Selective CO2 Sorption under Ambient Conditions in Seemingly Nonporous Molecular Crystal of Azacalix[5]arene Pentamethyl Ether

Described are the syntheses, crystal structures, and solid-gas adsorption behaviors of azacalix[4]arene tetramethyl ether and azacalix[5]arene pentamethyl ether. While the former compound exhibited no adsorption of four main atmospheric components, the latter selectively and rapidly adsorbed CO(2) at ambient temperature and pressure. X-ray crystallographic and potential energy distribution analysis revealed that azacalix[5]arene created an energetically favorable space for CO(2) in its seemingly nonporous crystal, leading to the observed selective CO(2) uptake under ambient conditions.

Crystallographic analysis of CO2 sorption state in seemingly nonporous molecular crystal of azacalix[4]arene tetramethyl ether exhibiting highly selective CO2 uptake

Described are the crystal structures and solid–gas sorption behaviors of azacalix[4]arene tetramethyl ether at low temperatures. Single crystals of the azacalix[4]arene with a seemingly nonporous crystal architecture exhibited highly selective uptake of CO2 among five examined gases such as N2, O2, Ar, CO2, and CH4. Single crystal X-ray crystallographic analysis successfully visualized the CO2 sorption state in which the azacalix[4]arene contacted with CO2 through intermolecular CH/O-interactions. A combination of the experimental results and theoretical calculations on the CO2 sorption state demonstrated that the observed high selectivity for CO2 was mainly controlled by a dispersion force, together with a molecular sieving effect.

Electric, electrochemical and magnetic properties of novel ionic liquid nitroxides, and their use as an EPR spin probe

Novel metal-free paramagnetic ionic liquid imidazolium compounds, which contain a chiral cyclic nitroxide unit as the redox and spin sources in the cation, have been prepared, and their first use as an EPR spin probe in diamagnetic ionic liquids as well as their electric, electrochemical and magnetic properties are described.