FORMATION OF INCLUSION COMPLEXES OF OLIGOETHYLENE AND ITS DERIVATIVES WITH ALPHA-CYCLODEXTRIN

Oligoethylene (OE) was found to form inclusion complexes with alpha-cyclodextrin (alpha-CD) not only from aqueous solutions of alpha-CD but also from DMF solutions of alpha-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 alpha-CD in DMF solution. The complexes were characterized by IR, H-1 NMR, C-13 NMR, and C-13 CP/MAS and PST/MAS NMR spectra, X-ray (powder), and elemental analysis. The continuous variation plots for the complex formation and the H-1 NMR spectra of the complexes show that the stoichiometry of the crystalline complexes is 3:1 (ethylene unit-alpha-CD). The X-ray powder pattern of alpha-CD-OE complexes and the solid state C-13 CP/MAS, PST/MAS NMR spectra suggest that the OE chain is included in the channel formed by alpha-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 alpha-CD. Derivatives of OE(6) with small end groups, such as hydroxyl, carboxyl and amino groups, formed crystalline complexes with alpha-CD, but OE(6) carrying large end groups, such as 2,4-dinitrophenyl and 2,4,6-trinitrophenyl groups, did not form complexes with alpha-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 alpha-CD, but can form 1:1 (guest molecule-alpha-CD) complexes which are in equilibrium in aqueous solution. The complexation of diammoniohexaethylene cation (DAHE cation) with alpha-CD in aqueous solution was discussed by using H-1 NMR and 2D NOESY NMR spectra.