The stastistical synthesis of rotaxanes is extremely inefficient. Therefore, non-covalent template effects are used to preorganize the rotaxane axle and its wheel in a threaded pseudorotaxane-like fashion. In the following step, bulky stoppers are attached to the axle in order to prevent it from deslipping. A mechanical bond is thus created.
We mostly use the crown ether/secondary ammonium ion motif and hydrogen bonded complexes of tetralactam macrocycles and amide or diamide stations. The group developed two new template effects, one based on an axle incorporating a phenolate anion, one utilizing a diketopiperazine axle center piece (see Figure). The diketopiperazine is held inside the wheels cavity by four hydrogen bonds.
Some of the rotaxanes are switchable. The phenolate axle can be protonated and deprotonated and then acts as a "molecular brake". When stopper attachment to the deketopiperazine axle is carried out by click chemistry, the wheel is located at the diketopiperazine station, but the addition of chloride shifts it to one of the two triazole stoppers.
Top and side view of a solid-state structure of a pseudorotaxane with a diketopiperazine axle threading through a tetralactam macrocycle