Controlling the pirouetting motion in rotaxanes by counterion exchange.

A fine control of the pirouetting motion of rotaxanes was achieved by using a series of metallabisdicarbollides. The latter were used as anions in the protonated form of benzylic amide macrocycle-containing fumaramide rotaxanes. The present paper discusses the synthesis and the structural and dynamic characterizations of the first examples of anionic boron cluster-containing rotaxanes. To study the dynamic properties of such molecules, the pirouetting rate of the weakly coordinating boron cluster-containing rotaxanes with the more strongly coordinating trifluoroacetate anion (TFA(-)), which would form a close ion pair with the macrocycle, was measured using the exchange spectroscopy NMR technique. Our hypothesis was that the stronger the ion pair the lower the rate of rotation due to the presence of a bigger volume of solvent to be moved. The anion would act as an anchor for the pirouetting motion. Indeed, the results show the expected trend: the rotaxane with the closely coordinating TFA(-) anion pirouettes most slowly, and the most weakly coordinating hexabromoderivative of cobaltabisdicarbollide pirouettes the fastest.