Akihiko Yamagishi

Molecular origin for helical winding of fibrils formed by perfluorinated gelators

A chiral low-molecular weight gelator, N,N-diperfluoroheptanoyl-1,2(R,R)- or -1,2(S,S)-diaminocyclohexane, was prepared to form a gel of acetonitrile. The conformation of the gelator in fibrils was determined by vibrational circular dichroism spectra, providing a molecular model for self-assembly in a helical fibril.

STEREOCHEMISTRY AND MOLECULAR RECOGNITION ON THE SURFACE OF A SMECTITE CLAY MINERAL

Attempts at optical resolution and asymmetric syntheses using smectite clay minerals are described. Use of the method was prompted by the discovery that the saturated adsorption of a tris(chelated) metal complex, [Ru(1,10-phenanthroline)3]2+, by Na-montmorillonite depended heavily on the stereochemical properties. The pure enantiomer was adsorbed by cation exchange at negative surface sites of the clay mineral, while the racemic mixture was adsorbed to two times excess of the cation exchange capacity. The chelate takes a uniform orientation on a clay mineral surface due to the matching between the molecular symmetry and the two-dimensional network of a phyllosilicate layer. On a clay mineral surface covered with the enantiomeric chelates, a vacant space capable of chiral discrimination was generated. Based on this, an ion-exchange adduct of smectite and the chiral chelate was used as an adsorbent for separating racemic mixtures or selectively producing either one of the optical isomers.

Chiroptical Studies on Supramolecular Chirality of Molecular Aggregates

The attempts of applying chiroptical spectroscopy to supramolecular chirality are reviewed with a focus on vibrational circular dichroism (VCD). Examples were taken from gels, solids, and monolayers formed by low-molecular mass weight chiral gelators. Particular attention was paid to a group of gelators with perfluoroalkyl chains. The effects of the helical conformation of the perfluoroalkyl chains on the formation of chiral architectures are reported. It is described how the conformation of a chiral gelator was determined by comparing the experimental and theoretical VCD spectra together with a model proposed for the molecular aggregation in fibrils. The results demonstrate the potential utility of the chiroptical method in analyzing organized chiral aggregates.

Theoretical study on vibrational circular dichroism spectra of tris(acetylacetonato)metal(III) complexes: anharmonic effects and low-lying excited states.

The open-shell density functional theory calculations with M06 exchange-correlation functional and all-electron Douglas-Kroll second order scalar relativistic correction were performed to interpret the vibrational circular dichroism (VCD) spectra of four kinds of tris(acetylacetonato)metal(III), [M(III)(acac)(3)] (acac = acetylacetonato, M = Ru, Cr, Co, and Rh). It was deduced that the experimental spectra were well reproduced by the calculation with harmonic approximation in case of [Co(III)(acac)(3)] (d(6); S = 0), [Rh(III)(acac)(3)] (d(6); S = 0), and [Ru(III)(acac)(3)] (d(5); S = 1/2). In case of [Cr(III)(acac)(3)] (d(3); S = 3/2), anharmonic effects should be taken into account to predict the accurate vibrational frequencies of closely located modes. Time-dependent density functional theory calculations were performed to estimate the contribution of excited states in the VCD spectra. As a consequence, the presence of the low-lying excited states was predicted for [Ru(III)(acac)(3)] alone, which agreed with the experimental observation.

Terminal effects on gelation by low molecular weight chiral gelators

Gel formation by low molecular mass gelators was investigated. The gelator was trans(RR)- or trans(SS)-N,N′-n-bromoalkanoyl-1,2-diaminocyclohexane (denoted as RR-CnBr or SS-CnBr, respectively; n = the number of carbon atoms in an alkanoyl group). When n was varied from 5 to 12, the gelators formed transparent or opaque or turbid gels in benzene except for n = 8. Focusing on the end effects, the gelation behavior of CnBr was compared with that of non-brominated counterparts (denoted as RR- or SS-Cn). From the vibrational circular dichroism (VCD) spectra of enantiomeric gelator/benzene-d6 gels, the signs of the coupled peaks around 1640 cm−1, which were assigned to the symmetric and asymmetric CO stretching vibrations from the higher to lower wavenumber, respectively, were opposite between SS-CnBr and SS-Cn (or between RR-CnBr and RR-Cn). On the other hand, the signs of the coupled peaks around 1550 cm−1 assigned to the NH stretching vibrations remained unaffected. The observed reversal of the signs in the CO couplet was rationalized in terms of the different modes of stacking in fibrils. In the case of Cn, for example, the molecules were stacked through anti-parallel double intermolecular hydrogen bonds using two pairs of >NH and >CO groups, while, in the case of CnBr, a single intermolecular hydrogen bond was formed with the remaining pair of >NH and >CO groups forming an intramolecular hydrogen bond. Single crystal X-ray analyses were performed for SS-C6Br and SS-C8Br. The results demonstrated that the length of the bromoalkanoyl chains drastically affected the packing modes in crystalline states.

Desorption of Cs+ ions from a vermiculite by exchanging with Mg2+ ions: effects of Cs+-capturing ligand

Cs+ ions intercalated in a South African vermiculite were so firmly bound that they were hardly dissociated on treating with an aqueous solution of 1xa0M KNO3 or NH4NO3. When a vermiculite having adsorbed the saturated amount of Cs+ ions (0.61xa0molxa0g−1) was stirred in an aqueous solution containing 0.4xa0M of Mg(NO3)2 and 20xa0mM of tetra kis(4-fluorophenyl)borate sodium salt (denoted as Na+L−), the intercalated Cs+ ions were fully replaced with Mg2+ ions within 2xa0h. The results were rationalized in terms of the ion-exchange reaction between Cs+ and Mg2+ ions:

Formation of chiral heterometallic oligomers by combination of inert Ru(III) tectons and labile Ni(II) connectors

2 {text{Cs}}^{ + } /{text{clay}} + {text{Mg}}^{ 2+ } ,,, rightleftarrows ,,,, 2 {text{Cs}}^{ + } + {text{ Mg}}^{ 2+ } /{text{clay}}

Application of Hectorite-Coated Silica Gel Particles as a Packing Material for Chromatographic Resolution

2Cs+/clay+Mg2+⇄2Cs++Mg2+/clay, in which the reverse reaction (or re-binding) of Cs+ ions was prohibited by the formation of an insoluble salt, Cs+L−.