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.

Stereochemistry and molecular recognition on the surface of a smectite clay mineral

Vibrational circular dichroism (VCD) spectroscopy was applied to study chirality recognition in the interlayer space of a clay mineral. Clay intercalation compounds including two kinds of chiral molecules were prepared. Firstly a cationic metal complex, Δ- or Λ-[Ru(phen)3]2+ (phen = 1,10-phenanthroline), was ion-exchanged into sodium montmorillonite. Thereafter a neutral organic molecule, R- or S-1,1′-bi-2-naphthol (denoted as R- or S-BINOL), was co-adsorbed. The solid state VCD spectra were recorded on the hybrid compounds thus prepared. The intensity of VCD peaks in the region of 1300–1400 cm−1, which were assigned to the bending vibrations of OH groups in BINOL, was remarkably dependent on the chirality relation between the two intercalated species. This implied that BINOL took a different conformation in response to the chirality of co-existing [Ru(phen)3]2+.

Solid state vibrational circular dichroism towards molecular recognition: chiral metal complexes intercalated in a clay mineral

Clay column chromatography for optical resolution: first clues, development and future

The chiral aspects of clay minerals are reviewed. First, the structural studies on the chiral crystal of kaolinite are presented. In order to see the chirality effects on stability, the first-principles calculation was performed to compare the formation energies of three polytypes of the kaolinite group. The chiral nature of a single crystal of kaolinite was studied by means of scanning electron microscope and back-scattered diffraction measurements. Second, the clay column chromatography for optical resolution is described. The method was based on the fact that the saturated adsorption amount of [Ru(phen)3]2+(phen = 1,10-phenanthroline) by sodium montmorillonite depended on its stereochemical properties remarkably. The maximum adsorption of the metal chelate was doubly different between the pure enantiomer and the racemic mixture. On a clay surface covered with the enantiomer of such chelates, a vacant space surrounded by chiral molecules provides a site for chiral discrimination. This situation lead to the use of an ion-exchange adduct of a smectite clay and the enantiomeric chelate as a packing material in liquid chromatography. The application was further extended to the asymmetric syntheses. Three examples are presented: the oxidation of sulfides photo-sensitized by [Ru(bpy)3]2+, the electrochemical oxidation of binaphthol derivatives on a thin film of clay/chelate adduct, and the enantioselective sensing by iridium complexes on a colloidal clay particle.

Chirality and Its Application

The adsorption of tris(1,10-phenanthroline)copper(II) ([Cu(phen) 3 ] 2+ ) by a clay mineral has been studied with electron spin resonance (ESR) measurements. The chelate, tris(1.10-phenanthroline)copper(II) ([Cu(phen) 3 ] 2+ ), is co-adsorbed with a large excess of racemic or enantiomeric [Ru(phen) 3 ] 2+ . The spectra of a freeze-dried sample of a clay-chelate adduct at 77 k show that there are two kinds of adsorption states: the one (denoted by state A) is characterized by the parameters of g II = 2.263 − 2.275 and A II = 151 − 160 G and the other (denoted by state B) by g II = 2.213 − 2.229 and A II = 150 − 186 G

Tokuko Watanabe

Papers

Electron spin resonance studies on tris (1, 10-phenanthroline) copper (ii) adsorbed by hectorite and saponite