NMR detection of simultaneous formation of [2]- and [3]pseudorotaxanes in aqueous solution between α-cyclodextrin and linear aliphatic α,ω-amino acids, an α,ω-diamine and an α,ω-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-cyclodextrin (alpha CD) were studied in aqueous solution by NMR spectroscopy. The association modes were established with titration and continuous variation plots, variable temperature NMR spectra, and dipolar interactions as recorded 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 simultaneously [2]- and [3]pseudorotaxanes with alpha CD in the aqueous solution. At the 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 constituent at both pH values. There were clear indications that the [2]pseudorotaxanes are present in two isomeric forms. The presence of isomers also in the [3]pseudorotaxanes was not ruled out. Various exchange rate regimes were observed; clearly in neutral solutions the formation of the 1:1 complexes was fast in the NMR time scale, whereas the threading of a second alpha CD ring was a slower process. In the solid state, the adduct of alpha CD/2 had the structure of a [3]pseudorotaxane, in accordance with previously solved 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 varying nature, and thus the frequently and conveniently assumed 1:1 stoichiometry in similar systems is an oversimplification of the real situation.