Controlling the Pirouetting Motion in Rotaxanes by Counterion Exchange

Abstract

KEYWORDS ROTAXANE • PIROUETTING • METALLACARBORANE • EXSY • MOLECULAR MACHINE 2 ABSTRACTA fine control of the pirouetting motion of rotaxanes has been achieved by using a series of metallabisdicarbollides. The latter have been used as anions in the protonated form of benzylic amide macrocycle-containing fumaramide rotaxanes. The present paper discusses the synthesis, structural and dynamic characterization of the first examples of anionic boron cluster-containing rotaxanes. In order 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, has been measured using the EXSY 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 the fastest.

Versatile control of the submolecular motion of di(acylamino)pyridine-based [2]rotaxanes

“…As we hypothesized, the protonation of the nitrogen atom of the pyridine ring seems to promote a conformational rearrangement 16 a of the axis 1c resulting into a clear steric interference of the ring rotation. 10c,18 These three switchable dynamic states are summarized in Fig. 1.…”

Carboranes in the chemist's toolbox

“…77,122 Rotaxanes Rotaxanes (mechanically-interlocked systems in which dumbbell-shaped molecules are threaded through macrocycles) are of considerable current interest as molecular machines, because their various modes of behavior including linear, rotational, and pendular motion are controllable. 124 In one such system (Fig. In a recent contribution Teixidor, Viñas and their associates used hexabrominated bis(dicarbollide) anions to control the pirouetting (rotational) motion of positively charged fumaramide rotaxanes.…”

Effects on Rotational Dynamics of Azo and Hydrazodicarboxamide-Based Rotaxanes

“…The peak for the equatorial methylene proton emerges as a doublet of doublets ( 2 J (H1,H2) = 14.3 Hz and 3 J (H1,HD) = 7.3 Hz) at 4.96 ppm and the peak for the axial one appears as a doublet ( J (H1,H2) = 14.1 Hz) at 4.06 ppm. This patent difference is connected with the rotational motion of the ring of single binding site amide-based [2]rotaxanes [ 16 , 17 , 29 , 30 , 31 , 33 , 34 , 35 , 36 , 37 ] which occurs at a higher spinning rate in [2H]- 3 than in 3 . As usually occurred in polyamide-based rotaxanes, peaks corresponding to the hydrogens of the thread are shifted to lower ppm values due to the presence of the macrocycle around it.…”

“…Indeed, the presence of electron-donor or electron-withdrawing groups can strengthen or weaken this interaction, breaking or accelerating the rotational motion of the ring [ 29 , 30 ]. In pyridine-containing macrocycles a chemical variation of the pyridyl nitrogen can also undergo changes on the rotational speed [ 31 ].…”