NMR detection of simultaneous formation of [2]- and [3]pseudorotaxanes in aqueous solution between alpha-cyclodextrin and linear aliphatic alpha,omega-amino acids, an alpha,omega-diamine and an alpha,omega-diacid of similar length, and comparison with the solid-state structures

The interactions of 11-aminoundecanoic acid (1), 12-aminododecanoic acid (2 ), 1, 12-diaminododecane (3), and 1,13-tridecanoic diacid (4) with alpha-cy clodextrin (alpha CD) were studied in aqueous solution by NMR spectroscopy. The association modes were established with titration and continuous varia tion plots, variable temperature NMR spectra, and dipolar interactions as r ecorded in 2D ROESY spectra. The studies were carried out at pH 7.3 and 13. 6. These long, linear bifunctional molecules were found to form simultaneou sly [2]- and [3]pseudorotaxanes with alpha CD in the aqueous solution. At t he higher pH the 1:1 adducts were present at concentrations higher than at the neutral pH. The longer guests formed complexes enriched in the 2:1 cons tituent at both pH values. There were clear indications that the [2]pseudor otaxanes are present in two isomeric forms. The presence of isomers also in the [3]pseudorotaxanes was not ruled out. Various exchange rate regimes we re observed; clearly in neutral solutions the formation of the 1:1 complexe s was fast in the NMR time scale, whereas the threading of a second alpha C D ring was a slower process. In the solid state, the adduct of alpha CD/2 h ad the structure of a [3]pseudorotaxane, in accordance with previously solv ed crystal structures of alpha CD/3 and beta CD/4. The species in solution, in contrast with those present in the solid state, are therefore of varyin g nature, and thus the frequently and conveniently assumed 1:1 stoichiometr y in similar systems is an oversimplification of the real situation.