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.