Kinetics of the self-assembly of bold alpha -cyclodextrin [2]pseudorotaxanes with polymethylene threads bearing quaternary ammonium and phosphonium end groups

Abstract

The kinetics and mechanism of the formation and dissociation of a series of [2]pseudorotaxanes, comprised of α -cyclodextrin (α -CD) as the cyclic component and the ([Me3N(CH2)nNMe3]2+ (n = 8-12), [Me2EtN(CH2)10NEtMe2]2+, and [Me3P(CH2)10PMe3]2+) dications as the threads, were determined by means of 1H and 31P NMR in aqueous solution. The length of the polymethylene chain (n) of the thread, which has a minor effect on the rate constant for pseudorotaxane formation, is important in the kinetics of the dissociation reactions, with the longer, more hydrophobic chains resulting in slower pseudorotaxane dissociation. The replacement of one methyl substitutent by an ethyl group in each of the end groups on the [Me3N(CH2)10NMe3]2+ thread results in a 30-fold decrease in the formation rate constant. Replacements, by ethyls, of two or all of the methyl substitutents prevent the formation of the pseudorotaxane, even after prolonged heating. The pseudorotaxane containing the {Me3P(CH2)10PMe3.; α-CD}2+ thread forms only at elevated temperatures by a slippage mechanism, and the rate constant for its self-assembly at 75°C (8 x 10-5 M-1 s-1) is more than 106 smaller than the rate constant at 75°C (200 M-1 s-1) extrapolated for the corresponding {Me3N(CH2)10NMe3 . α -CD}2+complex. The enthalpies and entropies of activation for the formation and dissociation of the [2]pseudorotaxanes decrease with an increase in the size and hydrophobicity of the end groups, suggesting a reduced role of desolvation of the quaternized atoms in the threading or dethreading processes.Key words: pseudorotaxane, α -cyclodextrin, kinetics, self-assembly, slippage, supramolecular.