An moving gif showing a rotating, entangled structure

Source: © Royal Society of Chemistry / CCDC 2100071

The molecule consists of two entangled gold–carbene metallotweezers (Au = yellow, N = blue, C = grey, H omitted)

A new member has joined the family of mechanically interlocked molecules. Its discoverers dubbed them clippanes since their chemical structure resembles that of two intertwined Keck clips.

According to the definition proposed by 2016 chemistry Nobel laureate Fraser Stoddart, mechanically interlocked molecules consist of two or more separate components that can only be untangled by breaking covalent bonds. One large class are ‘wheel-and-axle’ rotaxanes, which consist of a dumbbell-shaped molecule is threaded through a large cyclic molecule.

Now, a team in Spain designed a type of entangled metallotweezer – molecules originally created to trap planar aromatic molecules in the cavity between their two arms. Since the tweezers’ traditional U-shape complicates entanglement, the researchers crafted a more rigid alternative to meet the structural and electronic requirements of mechanically interlocked species.

An image showing an identical red and a blue structure locked together. Each part consists of a U bend with a long bar sitting perpendicular at each end of the U. The perpendicular bars of each structure is sitting behind the ones of the other.

Source: © 2021 Wiley-VCH GmbH

The molecule resembles two interlocked Keck clips

Each tweezer features two identical gold–heterocyclic carbene units linked with a pentacyclic aromatic spacer. The space between the arms – around 7Å – allows two tweezers to spontaneously self-aggregate. But when the carbene includes bulky tert-butyl substituents, the tweezer dimer cannot separate once assembled.

Clippanes offer chemists new ways to fasten molecules mechanically together, expanding the design toolbox for molecular machines.