Formation of Inclusion Complexes of Oligoethylene and Its Derivatives with α-Cyclodextrin

Abstract

Abstract Oligoethylene (OE) was found to form inclusion complexes with α-cyclodextrin (α-CD) not only from aqueous solutions of α-CD but also from DMF solutions of α-CD to give stoichiometric compounds in a crystalline state in high yields. The yields depend on the oligomeric degree (n) of OE when DMF was used as solvent. OE of n less than 6 did not form complexes with α-CD in DMF solution. The complexes were characterized by IR, 1H NMR, 13C NMR, and 13C CP/MAS and PST/MAS NMR spectra, X-ray (powder), and elemental analysis. The continuous variation plots for the complex formation and the 1H NMR spectra of the complexes show that the stoichiometry of the crystalline complexes is 3 : 1 (ethylene unit–α-CD). The X-ray powder pattern of α-CD–OE complexes and the solid state 13C CP/MAS, PST/MAS NMR spectra suggest that the OE chain is included in the channel formed by α-CD molecules and that the OE backbone in the complexes is more flexible than that in uncomplexed state (OE crystal) due to being included in the cavities of α-CD. Derivatives of OE(6) with small end groups, such as hydroxyl, carboxyl and amino groups, formed crystalline complexes with α-CD, but OE(6) carrying large end groups, such as 2,4-dinitrophenyl and 2,4,6-trinitrophenyl groups, did not form complexes with α-CD. However, it is found that derivatives of OE(6) with charged end groups, such as ammonio or carboxylato groups, do not form crystalline complexes with α-CD, but can form 1 : 1 (guest molecule–α-CD) complexes which are in equilibrium in aqueous solution. The complexation of diammoniohexaethylene cation (DAHE cation) with α-CD in aqueous solution was discussed by using 1H NMR and 2D NOESY NMR spectra.