Bistable Cucurbituril Rotaxanes Without Stoppers

Abstract

Bistable rotaxanes are important design elements of molecular devices for a broad range of applications, such as controlled drug release, molecular rotary motors, and chemical sensors. The host-guest complexes of cucurbit[6]uril and 1,4-bis(alkylaminomethyl)benzene were found to exhibit two stable binding modes with an unexpectedly high barrier between them. Their structural and dynamic properties, kinetic and thermodynamic parameters, as well as different chemical reactivity towards the azide-alkyne [3+2] cycloaddition reaction (click chemistry), were discovered by NMR spectroscopy, X-ray crystallography, and isothermal titration microcalorimetry. The highly stable 2:1 complex, which is formed at room temperature, was found to be a kinetic product, which may be converted to the thermodynamic 1:1 complex upon prolonged heating to 100 °C. The latter is a very stable rotaxane despite the fact that it lacks bulky end groups.

Attractive Interactions between Heteroallenes and the Cucurbituril Portal

“…Each of the protonated guest molecules, 2–6 , was dissolved in D 2 O-DCl at pH 5, then mixed with solid 1 (1 equiv) and the mixture was kept at room temperature for 16 h. Formation of 1:1 inclusion complexes was evident from their 1 H NMR spectra, which exhibited significant changes in the chemical shifts of the guest molecules in comparison with their spectra in the absence of 1 (Scheme 2 and Supporting Information). Consistent with previous observations, 13,14 all protons of the guest molecule residing in the host interior exhibited significant upfield shifts, which increased with the depth of burial in the binding cavity. For example, a comparison between free 5 and its complex 5@1 , revealed that the upfield shifts of the oligomethylene chain were shifted by 0.02, 0.52, 0.88 and 0.98 ppm, beginning with the α-methylene attached to the ammonium groups and moving inward.…”

“…We have previously reported that all guest protons that reside outside the cavity in the vicinity of the portal undergo deshielding, probably due to the strong anisotropic effect exerted by the combined dipole of the carbonyl groups at the portal. 13 Thus, in the case of the octamethylene chain of 6 , the lack of any shift exhibited by the α–methylene protons indicates that the shielding and deshielding effects completely offset one another.…”

“…13 The azide moiety of the guest N , N ′-bis-(2-azidoethyl)-p-xylylene diammonium dichloride forms close contacts with the oxygen atoms of the host, and the azidoethyl group adopts an energetically unfavorable gauche conformation (Figure 1). These observations suggested an interesting stabilizing attraction between the azide group and the carbonyls.…”

Binding of carboxylatopillar[5]arene with alkyl and aryl ammonium salts in aqueous medium

“…6c Guest molecules G1-G4, 25 G5 30 and G6 29 were prepared according to the reported literature. Host molecule CP5A was prepared according to the reported literature.…”

Rotaxanes and pseudorotaxanes with threads containing viologen units

“…32 There should be mentioned here also rotaxanes and pseudorotaxanes containing cyclodextrins, [33][34][35] calixarenes 36,37 or cucurbiturils. 38,39 Today many works concerning viologen-based rotaxanes and pseudorotaxanes appear, [40][41][42][43][44][45] in these systems viologens are components of a thread [45][46][47][48][49] or are present as parts of the tetracationic cyclophane serving as a ring. [50][51][52][53] Moreover viologens find a variety of applications due to their redox properties, they are promising for design of molecular machines and switches and construction of new materials, the rapid development of viologen chemistry has its reflection in many reports concerning these compounds.…”