Transition-metal-complexed catenanes and rotaxanes in motion: Towards molecular machines

In the course of the last decade, many dynamic molecular systems, for which the movements are controlled from the outside, have been elaborated. These compounds ware generally referred to as "molecular machines". Transit ion-metal-containing catenanes and rotaxanes are ideally suited to build such systems. In the present review article, we will discuss a few examples of molecular machines elaborated and studied in Strasbourg. In the first section we will discuss an electrochemically driven system, consisting of copper-complexed catenanes and rotaxanes of various types, including a fast-moving pirouetting rotaxane. The second part will be devoted to chemically driven dynamic molecular systems. A porphyrin-stoppered rotaxane, able to undergo a pirouetting motion by metallation or demetallation of the central coordination site, and a linear rotaxane dimer whose behavior is reminiscent of muscles, will be discussed. In both compounds, the mobile component(s) will be set in motion by modifying the central coordination of the molecular assembly. In the rest of this review article, we will mostly focus on light-driven machines, consisting of ruthenium(II)-complexed rotaxanes or catenanes. For these latter systems, the synthetic approach is based on the template effect of an octahedral ruthenium(II) center. Two 1,10-phenanthroline ligands are incorporated in an axis or in a ring, affording the precursor to the rotaxane or the catenane, respectively. Ru(diimine)(3)(2+) complexes display the universally used (MLCT)-M-3 (metal-to-ligand charge transfer) excited state and, another interesting excited state, the (LF)-L-3 (ligand field) state, which is strongly dissociative. By taking advantage of this latter state, it has been possible to propose a new family of molecular machines, which are set in motion by populating the dissociative (LF)-L-3 state, thus leading to ligand exchange in the coordination sphere of the ruthenium(II) center.