Molecular machines and motors based on transition metal-containing catenanes and rotaxanes

Molecular motors of various kind (linear, rotary) are very common in biolog y where they play an essential role. However, the number of synthetic molec ular ensembles whose dynamic behavior is reminiscent of biological motors i s presently very limited. In order for an object to be regarded as a motor, several basic requirements have to be fulfilled. Even without trying to ap ply a strict thermodynamic definition, the system will have to convert a ce rtain type of energy into another form of energy, while undergoing some kin d of continuous motion. Threaded or interlocked rings are ideally suited to the construction of fully artificial molecular motors. If a ring is thread ed onto a rod, it can either rotate around the axle or undergo a translatio n movement. Similarly, in catenanes, a ring can glide at will within anothe r ring. Several examples of such compounds have been elaborated and studied in recent years, using threaded and interlocked molecules either based on acceptor-donor and hydrogen-bonded complexes or on transition metal complex es.