Mechanically interlocked molecules such as rotaxanes and catenanes comprise two or more components whose motion relative to each other can be controlled. A [2]rotaxane molecular shuttle, for example, consists of an axle bearing two recognition sites and a single macrocyclic wheel that can undergo a to-and-fro motion along the axle-shuttling between the recognition sites. The ability of mechanically interlocked molecules to undergo this type of large-amplitude change is the core mechanism behind almost every interlocked molecular switch or machine, including sophisticated mechanical systems such as a molecular elevator and a peptide synthesizer. Here, as a way to expand the scope of dynamics possible at the molecular level, we have developed a molecular shuttling mechanism involving the exchange of rings between two recognition sites in a saturated [3]rotaxane (one with no empty recognition sites). This was accomplished by passing a smaller ring through a larger one, thus achieving ring-through-ring molecular shuttling.

Shuttling rates for neutral and charged [2]rotaxanes with rigid axles varying in lengths from 7.4 to 20.3 Å were found to be independent of the length of the axle, except when the distance was short enough to allow the ring to interact with both recognition sites which provided a short-cut mechanism that significantly lowered the energy barrier.

A novel [2]rotaxane was constructed that has a bidentate N,N'-chelate as part of a rigid, H-shaped axle and a 24-membered crown ether macrocycle containing six ether O-atoms and an olefinic group as the wheel. This unique topology produces a ligand with the ability to dial-up different donor sets for complexation to metal ions by simply rotating the wheel about the axle. The solution and solid-state structures of the free ligand and complexes with Li(+) and Cu(+) show how the ligand adopts different rotational co-conformations for each. The Li(+) ion uses the N,N'-chelate and O-donors while the Cu(+) center is coordinated to both O-donors and the olefinic group. This concept of rotationally active ligands should be possible with a wide variety of donor sets and could find broad application in areas of coordination chemistry, such as catalysis and metal sequestration.

A benzimidazole molecule can be protected from deprotonation with strong base by converting into a suit[1]ane – a type of 3D mechanically interlocked molecule. Utilising a combination of ring-closing and ring-opening metathesis, the wearing of the protective “suit” can be made reversible.