Bistable cucurbituril rotaxanes without stoppers

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. Locked up and active: The host-guest complexes of cucurbit[6]uril and 1,4-bis(alkylaminomethyl)benzene exhibit two stable binding modes with a high barrier between them. The highly stable 2:1 complex that is formed at room temperature is a kinetic product that may be converted into the thermodynamic 1:1 complex upon prolonged heating in water (see scheme). The latter is a very stable rotaxane despite the fact that it lacks bulky end groups. These findings are significant for the design of molecular devices.